Hmmm I know I couldn't relogin to yahoo mail until I disabled a couple addons (then enabled them once again when it accepted me); Probably because Yahoo does advertising the addon(s) rejected, as GoG didn't give me those issues...

Recurring server-side problem. Several threads have documented the footprints of it. GOG should ask for specific info if they need more and otherwise fix it.

I have the same problem. I'm using Chrome without extensions, and I stay logged in on forums, blogs, shops, social networks, Steam client, etc. And I keep Chrome and my system always updated.

I'm signed in, but can't access my library, as there is [sign up | log in] instead of Account

MMMmm ^^

Tin Foil Hat /on

Maybe the sign up \ log in buttons are fake, and that's how peoples account gets compromised

Tin foil hat /off

Same here and it started a week ago or so.

The same problem for me. It has started about a week ago. I haven't changed my Firefox settings and I have no problem with other sites.
On the other hand, Opera doesn't apper to have this glitch, so maybe a new Firefox update has broken something compatibility-wise.

I`ve had this happen two or three times in the past, no idea what causes it but it`s not at my end.

Fix:

Close your browser, use something like CCleaner to clear cache/cookies et al.
Start browser again & with any luck, should fix it.

Just done this & it worked for my browser (PaleMoon).

I shut down firefox, re started, logged in, now I can access my library.

Web-based applications are very subjective, especially when browsers do different things at different times and for those who have theirs set differently as well.

Clear the browser cache and cookies. (backup your important cookies, except those from the site you are trying to get a 'reset') Also make sure you remove everything related to the site from "local storage". I believe once you do that, you'll be OK.