I have the Red Goddess, Onoskelis, Demon of the Past, uncensored content patch and Elysian Fields.
They don't appear ingame.
The uncensored part got fixed thanks to another thread here in the forum by downloading the zip file directly from the website.
For the other dlcs, I tried to install them manually, but the installer says "Please update your game version with the latest patch.".

Should I downgrade the game to an older patch version? Is it even possible to do this?

When rightklicking the installed game inside GoG-Galaxy, you can choose "configure installation" and choose the version before the latest one. Funny the latest version is called "no_dlc", so no wonder they don't work.
Starting download will be roughly double the size of the final version.

Thank you, it works! I will stay on the previous version until the next new update will fix it. :)