skowronek: Chiming in here as this also worked for me.
1. Shutdown all instances of any GOG application or service
2. Copy/backup %ProgramData%\GOG.com\Galaxy\storage\galaxy-2.0.db > galaxy-2.0 Copy.db
3. Download and install a SQLite IDE (I used HeidiSQL)
4. Connect to galaxy-2.0.db
5. Run the recommended SQL query below
6. Launch GOG
7. Profit!
INSERT INTO ProductConfiguration(ProductID,createdAt)
SELECT productID, CURRENT_TIMESTAMP
FROM InstalledBaseProducts
WHERE NOT EXISTS (SELECT 1 FROM ProductConfiguration WHERE InstalledBaseProducts.productID = ProductConfiguration.productID);
Wow! I didn't expect anyone to edit the database directly. Not bad :)
As always, though, be careful with such things (that point about backing the DB up is very good) as it may screw things up for GOG GALAXY.