I have come back to playing Heroes III after a long break, around about half a year. At this time, multiplayer worked with gameranger fine. Now suddenly, what I get whenever I try to join is the main screen but with a loading hourglass. After a minute, it exits the game. There is an error message but it's only up for a microsecond so I can't even read it.

This seems to be a common issue with people using gameranger and the complete edition of Heroes III from GOG. I have looked everywhere for an answer but can't seem to find one. I am using Windows 7 64 bit, all the necessary ports are open and all the exceptions rules were already done for Windows firewall. I even tried turning the firewall off, still the same issue.

Other games work fine on gameranger, so this seems to be H3 GOG-edition specific. I am stumped here, does anyone know what it could possibly be? Did anyone else have the same problem? Thanks!

No one? :(

Do you have any mods attached to the game, such as the HD mod, Wake of Gods (WoG) or Horn of the Abyss (HotA)?

I also have the same issue (together with my friends that are trying to play, as well as between my two personal computers). I'm also using w7 64bit.
I have tried with HD+ (I did actually get it to work once with a friend a few months ago) and without HD+ without results.

Just for the sake of it I will describe my installation process in case someone notices something off.
Game --> universal update --> HD+, create HD exe from hd launcher and rename it to the same name as the original heroes exe. (So that gameranger launches that instead)

IIRC I read somewhere that there supposedly is some issue with the universal update and HD+, I have not tried without it yet, but I will do that now. I will report back if I find something.

After some testing it seems like it's the universal update that breaks things for me.
If I don't install the universal update I can connect to the server, with and without HD+.

Ok, so I tried again today, and now it doesn't work... I have changed nothing. I tried again because I wanted to test it out with a friend and not just my own laptop.
Looks like it works when it feels like it...

Back to pulling my hair.

EDIT:
I decided to edit my old post instead of posting a new one so I don't spam as much.

Today's findings are better, but not that good. I noticed a window popping up as the game shuts down when I connect. I couldn't read it at first as it disappears immediately (which is why I didn't see it in the first place). But I managed to get a screen shot of it by recording my desktop, it says "Sorry, there was an error connecting to that session. Please try again.". Upon searching some more with this message I found that it could be a firewall issue. I never really thought about that since I never really have had a problem with that before, but I checked it out, the windows firewall seemed alright. But I tried to connect again with my firewall disabled, no dice. Still not connecting.
This may however fix the issue for others.

After I tried that I just reconnected to the server over and over, getting the same error, while I was thinking what my next move would be. Suddenly I'm connected again. So it really does work when it feels like it, I tried to connect again after that, after a few tries I manage to get connected again.

Maybe it's possible to get a game together by just brute forcing, I don't know, but I will probably try.

So to summarize:
I am unable to connect to a server, the message I get is "Sorry, there was an error connecting to that session. Please try again.".
(This is an issue that can sometimes be fixed by opening up the windows firewall / port forward according to http://heroescommunity.com/viewthread.php3?TID=12141 )
I am sometimes able to connect by trying to connect over and over.

It would be nice to at least be able to find why things aren't working as they should, and eve better, find a solution for it.

I'll be back if I find something new.

Nothing new?
I have the exact same problem. Me and two friends had it working for 2 days until it stopped working today. After restarting all our computers it worked however when we tried to start a new game a bit later it didn't work again.
Me and one of the friends have the exact same problem described here. Both of ous had the HD mod. I did however uninstall the HD mod and reinstalled H3 to no sucess.

Sadly not, though I haven't tried in quite a while now. I might pick it up and try some things again soon as I and some friends started talking about it again.

I've finally been able to connect to my own game through Gameranger from my laptop to my desktop pc! (Hurray!)

What I had to do (which I though I had already tested, but must have done incorrectly somehow) was to port forward UDP port 16000, as Gameranger suggests when it complained about me being behind a firewall (I said "Port-Restricted Cone NAT Router"). As I said, I tried that before with no luck and some time after that I think I enabled DMZ on my router, which I thought would let me get away with not forwarding any ports (it worked fine with other games when I enabled DMZ). But I might have misunderstood what that actually does since it seems I still need to port forward for Gameranger.

To summarize exactly what I have done:
I installed HoMM3 from GoG Galaxy (It seems to have been some update that combined the universal update or something, because I could'nt find it anymore anyway).
I enabled DirectPlay manually, no window popped up telling me I needed it (even though I don't think I needed to do this since I was still able to host a game through TCP/IP, which would be impossible without DirectPlay as you can't even enter the menu if you don't have it, more about his below).
I port forwarded the UDP port 16000 as suggested by Gameranger.

Anyway, I thought that I should share all the other tips that I found (and have tested) that has worked for other people. I will list this with a brief description below:

1. Installing DirectPlay, this is something some older games use and it can be installed by going to "Turn Windows features on or off" in the control panel, expand "Legacy Components" and tick DirectPlay, then jsut click Ok and it should install. (Comment about this: When I launched HoMM3 on my laptop a window popped up telling me I should install DirectPlay, while it never did on my desktop pc and seemed to work fine anyway. My desktop pc runs Windows 10 Home 64bit and my laptop runs Windows 10 Home 32bit. Some more comments about DirectPlay in the next point below)

2. Removing the DPWSOCKX.DLL file, to me it sounds like the poeple from the source find the file in the HoMM3 installation folder somewhere, but I might be misunderstanding that. Anyway I found it only on my laptop in the system32 folder. Now removing files in system32 can be dangerous, and you might need to change some access rights to be able to remove it (I won't be describing how to do that here, partly because of the possible danger in doing so and also because it's quite easy to google, just google something like, "how to i remove something from system32") From what I can find the DPWSOCKX.DLL file comes from DirectPlay and handles the TCP/IP part of the game it seems, because when I removed it from my laptop I was unable to click the TCP/IP button after, I got some error message that TCP/IP was unavailable or something like that. But as I said, I could not find this file on my desktop and entering TCP/IP in the menu works fine there.
Source: http://heroescommunity.com/viewthread.php3?TID=12141&pagenumber=36

3.Disabling UPnP on your router , I tried it but it didn't change anything for me, so I enabled it again.
Source: https://www.gog.com/forum/heroes_of_might_and_magic_series/homm3_and_tcp_ip_lan_problems

Now I haven't extensively tested my solution, but I will try to get a friend to test a few turns with me tomorrow (as I've heard that some have crashes after some turns), I also haven't tested the HD mod, but I honestly don't see any reason for it to not work, I will probably use it when we finally decide to play if everything seems to to be working fine.

I hope this summation will help someone find a solution to their problems.

Good luck!

Regards,
Kurappu

What has helped me was going and setting the heroes3.exe to compatability mode windows XP SP 2, and setting it to launch as administrator.

Whenever I try to play with a friend I dont show up on his game, the game launches from the host screen and everything looks fine but it's like we're playing separate. Funny thing is (not funny) this issue happened to us some time ago and we tried everything and we solved it, but now we don't remember (I know it's ridiculous but we tried so many things we don't remember which one worked out and we forgot many of them).