Telian: I had the same problem. Reinstallation showed then that my virus protection program didn't like the new launcher version that should have been installed during the update, and put it into quarantine. It was labeled as containing a gen.6 trojan virus ...
Same problem with Riven, but with a new installation. I'm using Norton Security version 22.14.2.13. Relevant security log entries are below.
Category: Resolved Security Risks
Date & Time,Risk,Activity,Status,Recommended Action,Path - Filename
7/14/2018 8:12:08 PM,High,launcher_riven.exe (Trojan.Gen.6) detected by Auto-Protect,Quarantined,Resolved - No Action Required,d:\gog\gog galaxy\games\riven - the sequel to myst\launcher_riven.exe
Category: Resolved Security Risks
Date & Time,Risk,Activity,Status,Recommended Action,Path - Filename
7/14/2018 8:12:07 PM,High,launcher_riven.exe (Heur.AdvML.C) detected by Auto-Protect,Quarantined,Resolved - No Action Required,d:\gog\gog galaxy\games\riven - the sequel to myst\launcher_riven.exe