Yesterday, the game (patch 5, Hotfix 2) crashed, when I was leveling up a character. Patch 5 had worked fine until then. I sent a bug report (which strangely took about 10 minutes to send). When I started the game again (right afterwards), it made me sign the EULA (again) as well as two new agreements to give Larian access to a lot of private data. I also got several pop ups like "we inform you that this game contains nudity, do you want to enable this feature" although I made neither updates nor new installs.
My savefiles did not work anymore, so I started over again and ever since, the game resets the options every time something happens (f.ex. the hotbar and subtitles snap back to default every time I exit the menu) and the tutorial resets every time I press a button (f.ex. using and exiting the map gives me the map tutorial every single time). Many of my dialogue Options are "1. Continue" (which I'm used to from Act 3 but not right at the beginning of the game) and a lot of other stuff like f.ex. voiceovers do not work propperly. Starting the games takes two to three minutes now (usually it was like 30 seconds).

What did not stop the issues was:
Installing new
Installing older versions of the game
Deleting all saves and starting over (also with older versions of the game installed)
Deleting all GOG-related content from the computer.
Deleting all Larian-related content from the computer (as far as I could find).
Using all the "data mismatch" solutions provided by Larian (I get the data mismatch-pop up; always did; I was told to ignore it)

I do not use GOG-Galaxy.
I can root out a virus. (just in case someone's asking)
Bug-Situation is the same for Balanced, Tactician and Honour-Mode.

Also - not to rant about the the game's quality, but I'd be interested since I start to get highly frustrated at this point: I invested about 300-400 hours into the game by now and I did not make it far past the Lookout at the beginning of Act 3 yet. Past Act 1 I always encounter all sorts of gamebreaking bugs until the game just stops working (stops starting dialogues or/and making quest updates). Friends told me that it was only me, but non of them have played past Act 1 by now. Is this common or is it just my luck? My PC is up to date and I normally have no trouble running programmes or games, especially GOG-products.

Thanks in advance for any suggestions!

I use Galaxy, but I've successfully played through to the end, and I think I only encountered one or two bugs that required me to revert to my last save. I finished every character's side quest except that dark elf pally... got a pretty solid run through it all. I probably benefited from a few now-fixed bugs about people falling off cliffs to die during key battles, but they didn't seem like bugs at the time.

I did have to step down to medium-quality shadows in ACT 3 due to the game getting laggey the longer I played, but other than that it seemed to work generally smoothly.

That said, I did that a few patches ago. I can't say for certain that the newest patches didn't break something. Seems unlikely that they'd regress that much, but you never know.

... just one data point.

Yikes. Yikes because you probably did too much and at some point might have messed up with something.

What you experienced in the first lines of your description is your game switching to a debug profile.

Such game behaviour happens often with a bad Windows setup and can also rarely happen with a Mac.

Hence one of my most common recommendations at release was: if it crashes, restart your PC.

If the issue persists like you described it further, it could be a result of data corruption. Generally speaking not just savegame corruption, but data generation corruption in general. Again: caused by crashes or bad OS setup.

Reinstalling the game will rarely ever help, because the data corruption does not happen with the native game files but the game files which are additionally generated by the game.

It has nothing to do with GOG related files.
You probably missed Larian-related files - but even then you do not need to go that far to delete your savegames unless you are absolute certain your savegame are affected by corruption.

The benefit of using GOG Galaxy is that you can verify the game's files. But that is 100% not necessary unless it comes to regular patching. When you suspect something could have messed up by patching, you either install GOG Galaxy -> it recognises the game -> you run a verification -> you can delete Galaxy again.

Alternatively: download the latest offline installers and reinstall the game fresh while also cleaning the game cache. Done. No need for GOG Galaxy.

The initial issue you described: crash at level up, is rare and every case related to issue with graphics cards software any anything which can meddle with that such as overlays. Aka discord for streaming or GeForce Experience for quick recording.

Most players were successfully working around that by disabling any interfering software such as Adrenaline / GeForce Experience auto optimisation and using the official Larian Support's troubleshooting profile. Running the game in windowed mode plus using Borderless Gaming makes it more stable altogether.

Here is what you take from this

1. If your game crash -> restart your PC
https://steamcommunity.com/sharedfiles/filedetails/?id=3016984650

2. Clean your Cache if you encounter glitches, especially before/after patching the game
https://steamcommunity.com/sharedfiles/filedetails/?id=3018203217
(ignore the steps with Steam there - reset game e.g. does not work for you unless you use Galaxy)

3. Check your OS' settings if you want to reduce the possibilitiy of issues you have
https://steamcommunity.com/sharedfiles/filedetails/?id=3065050571

4. If the issues persists which are rooted with graphics cards software consider using Display Driver Uninstaller (DDU) for a fresh install and try out the troubleshooting profile by Larian: it reduces the graphics settings that much that it can tackle many related issues. Try also to run with any software off which could interfere. There are plenty such as Afterburner, Process, Lasso, Adrenaline/Geforce Experience, discord, overwolf, and so on.
https://steamcommunity.com/sharedfiles/filedetails/?id=3025213577

5. If you ever want to make a clean install, you must make a clean uninstall first. Best if you use an app like RevoUninstaller (the free version / portable suffices) - the advanced options of such an app searchs actively for registry entries and files you are missing. Careful with your savegames not to accidently delete them

With that toolset you should be good to go. Good luck!