Hi,
today I had "Darkside Detective" update and after that I've lost all my progress. Today I finished Case #4 "Police Farse" and wanted to start next case.
When I opened game after update I noticed some additional developer logo "Akupara Games" (or I didn't notice that earlier) and now I see only first two cases available to play without my previous progress.
When I checked seve files, then noticed files like:
"AC_The_Darkside_Detective_Options.LEGACY_SAVE_IMPORTED_2021-9-11_SOME_NUMBER"
and
"The Darkside Detective_666_0.LEGACY_SAVE_IMPORTED_2021-9-11-SOME_NUMBER"

How can I restore my saved progress?
Someone had the same or simillar issue?

Same here!

Luckily reverting to the previous version using Galaxy worked. If you are using offline installers, try reinstalling from the previous installer you used.

There's no changelog for the new version and nothing to suggest why you'd need the update, so for now I recommend staying on the previous version.

I've tried reverted game version in GOG Galaxy (from v. 2.3.0.1496d to previous version gog-7 Update 5), but I still don't have saves back...

I don't know what to suggest then, sorry! Maybe the cause for your save disappearing is different from mine.