Solved in the end!
Well, it's kind of irritating. Is it Steam or Windows 8 or both?
After years with WinXP I am with Windows 8 now (Once you get rid of Metro with either Start8 of Classic Shell, it's ok actually).
Now I get the same error window as previous users.
And same warning, when starting new BUFFY game without anything wrong in settings screen.
Uninstalled BTS and Steam from C:\Program Files (x86). Created new folder on drive E:\Games\ with all permissions and installed Steam and BTS there.
I run as Administrator but just in case granted Full Control permission to Everyone to Steam folder and sub-folders.
Cleared this stupid Read Only attribute in folders properties (which comes back on its own BTW).
Made the same with C:\Users\...\Documents\My Games folder.
Buffy path:
looks perfectly right.
Did not help.
Even more, there is only 1 login on my PC - administrator - and it's me, but ok, I go to compatibility and check run as Administrator.
Then, in game I go to Advanced -> load mod -> BUFFY 3.19.003 and get this nice thingy:
You get this, after applying
any of run as Administrator,
or Run this program in compatibility mode for: Windows XP Service Pack 2 / 3.
Hilarious (NO)!!! Either Steam closing BTS to slowly, or Windows 8 starting BTS too fast, but indeed, new game process try to start while previous process haven't yet been terminated. This happens with trying load any mod, not just BUFFY.
What did I miss? Cuz right now I consider to just download
non Steam version of Civ4 Complete from Torrent. After all I paid for this game long ago, but can not use it properly for some stupid reason. BTW there is minor problem with Steam version where double clicking save file in windows explorer does not know what to do with the file, you need specify open with
path to BTS.exe, then it will open the game but you will need to load save manually.
While typing this post I applied Run this program in compatibility mode for: Windows 7 as last resort.
And it worked!
I hope this will help future generations.