Cannot upgrade / permission denied when joining a game

[watermelon]

I play FAF at my computer without issues, but when my son tries to play on his own Windows account and his own FAF account, he gets the following error when he tries to join a game:

Files to update: [ForgedAlliance.exe, init_faf.lua, init_ladder1v1.lua, GDFBinary.dll, SupComDataPath.lua, SupComDataPathFAF.lua, init_nonxt.lua]

He is a member of the administrators group, and I also tried to grant him rights via folder permissions/security but it didn't help.