I compared the GOG version
with the version available at Zoom Platform
(both DRM-free versions of the game). In the GOG version some of the mansion cutscenes are not working as we already know. In the Zoom version however all cutscenes are playing 100% fine
Because of that I tried to replace the GOG avi files with the Zoom avi files but still in the GOG version the same "The system cannot find a decompressor that can decompress this movie"
message appeared. So it can't be a problem with the avi files. It must be some problem with an ingame trigger not working correctly. I'm not sure if that could be fixed at all.
For some unknown reason I was not abe to record videos or create some screenshots from the game but here is an older video of the Zoom version 1.2 (still the current version) showing one cutscene that doesn't work in the GOG version: https://youtu.be/BuwybJ-qi-Y?t=13m41s