Still better not to idle CSGO or any multiplayer game when connected to a server, there is some risk of a sanction there even if it's not a vac ban.

Or Archi's steam farm, that's pretty robust (but slow)

Or idle daddy on android

Try IM extended 1.4 instead if u can.

And if it's not working then see:
psteamcommunity.yuanyoumao.com/groups/idlemastery/discussions/0/133257636766989675/

And it is steam sale time so it could be a steam server issue.
Are you logged in to the steam client?

Use Idle Master Extended
https://github.com/JonasNilson/idle_master_extended/releases

If you're using windows then use Idle Master extended, doesn't require any cookie modification.

What OS are you using and which version of Idle Master?

steam-idle.exe doesn't normally close until you close it AFAIK so it may be the start command times out, IDK.
I see there a /wait parameter to START, you could try that.

Tested, doesn't work, 6 instances of a game for half hour and only half hour counted.

Steam only accepts first 30 games AFAIK, the rest are ignored. As for the games being the same, My guess is it would only count 1 instance, you could try it with 5 and see what happens.

Steam servers hate too much of anything, there is rate limiting everywhere - view too many store pages = temporary block, view too many community market pages = temporary block etc.