Hello,
I will try to give as detailed information as I can.
BACKGROUND:
I have installed GOG Galaxy 1.2.57.74. After changing my PC with a newer one, I imported my existing GOG games into the client. All of them worked (and still work) fine. Suddenly, I found that in Titan Quest Anniversary Edition the two DLCs - Ragnarock and Atlantis are not recognized in the game. In GOG Galaxy they are indicated as installed, but in the game, on the main screen I am invated to purchase Ragnarock. When I click the link on the screen, It redirects me to GOG where the only available option is "purchase as a gift". The directory where my GOG games are installed is named in Cyrillic "Игри" = Games so, I suspected that this may play role in this problem. I renamed the folder using latin letters and then re-imported the games. When I started Titan Quest Anniversary Edition this time, both DLCs of the game were detected and the link invating me to purchase Ragnarock disappeared. I thought - OK, the problem is solved.
CURRENT PROBLEM:
Yesterday I received an update of Grim Dawn (1.1.4.2 one) which started instalation autamaticaly, but the instalation failed on the configuration step (INSTALLATION FAILED: CONFIGURATION ERROR). So, I manage the instalation of the game => Verify / Repair. GOG Galaxy re-downloaded the update and procead to instalation and again failed on the configuration step (the same message as shown above). Then, I decided to uninstall the game, and I remove it using GOG Galaxy uninstall option retaining the saved games. Then, I found that despite the game is removed from GOG Galaxy list of installed games, all the game files are retained in the installation directory - they are not deleted. I deleted the folder of the game manually and proceeded to a new download and installation of Grim Dawn with all the DLCs. After the installation step, GOG Galaxy continued with configuration of the game and failed again on this step. The "Play" button got inactive and GOG Galaxy entered in perpetual loop trying to verify and configure the game. I uninstalled the game again using Uninstall option of GOG Galaxy. The game was again removed from the list of the installed games but its instalation files were untuched. So, I imported the unconfigured folder in GOG Galaxy and it was recognized. I can play the game and all the saves and settings of the game are retained. However, if I try to Verify / Repair instalation of the game, the configuration error appear again and the game is unplayable untill I "uninstall" and then import it. The situation is the same with all other games which I have currently installed installed in GOG Galaxy (7 games).
In addition, when I "uninstall" some of these games and the "INSTALL" button gets active. When I press the button a dialogue appears to choose an installation path. I choose the same path as this of the "uninstalled" game (all of its files are already there) and GOG Galaxy proceed downloading and instalation without noticing the available files - so an instalaltion in the installation appears, and again the installation errors at the configuration step.

In the settings of GOG Galaxy "Auto-update games" is enabled, and "Cloud saves" option is disabled (by me).
OS is Windows 10 Pro.

I will be glad if you have some suggestions concerning the above-mentioned issues.

One suggestion, download the backup installers and install from that, then import to Galaxy if Galaxy doesn't see it. That's a work around, for now at least. Have you tried reinstalling Galaxy?

Or just use the normal installers that don't rely on galaxy, and play your games that way.

OK, I re-installed GOG Galaxy and also several of the games from backup files. The same issues appeared when I performed Verify / Repair step with one new one: Installation Failed: Disk Access Problem. This one tends to appear randomly usually followed by the configuration error mentioned above. I run GOG Galaxy as an administrator. I've deleted $AV_ASW folder (some comments from other topics suggested this) which was from Avast. Currently, I have Kaspersky Free, and the mentioned folder was from previous installation on my older PC and is located on a different partition than the System one where GOG Galaxy is installed. As I've mentioned, the games are completely playable but in this state GOG Galaxy is useless - not able to install the updates properly, nor new games or to repair them. Very unstable behavior which on a first sight comes from nowhere...

So why don't you use the normal installers that don't need galaxy?
You don't need galaxy to play your games.

Have you installed your games in a Windows folder? (like Program Files)
Don't!

(btw to be safe I'd manually backup my saves\settings before messing with Galaxy)

@FrodoBaggins - Auto update; faster installation of the games from my library.
@teceem - Nope.
@phaolo - My saves are secured.

I do not exclude the possibility the problem is on my TV only. Waiting for a new GOG Galaxy update. Till then - I am using the standard launchers of the games.