Maybe I'm just stating something wrong, but if I was able to get in the server it's probably because Tim and other drivers on srouk have a different file from the original in game.
My reasoning is this. I had an almost fresh install of AC. I just dragged Sierras cars folder form the link provided by Tim (Race Department) into the cars folder. Provided that me and Tim both downloaded the same cars form his link of post 1, Sierras shouldn't be the issue. So the issue was probably related to BMW, Alfa or Merc, or, alternatively, to the Zandvoort track, of which Tim uploaded aswell his folder.
That must have been a different file inside one of that folders. Did you have some race with DTM mod in the recent past messing around with files?
It's really weird, cause I erased the Alfa, BMW, Merc and Zandvoort folders and run the integrity check, that should have re-installed the original files. So, maybe, I'm wondering if it's something related to a race that you had in the past here on SROUK.
I'll do this test: I will delete one folder at a time, then run the integrity check to let the game re-install the original files, and see which folder creates this issue. Please leave your server up for a few hours on Zandvoort. Thank you
Did the test, and the trouble is surely related to the folder Ks_zandvoort.
I deleted the track folder sent by Tim and run the integrity check. After re-running AC, I received again the message PROBLEMS DURING CHECKSUM.
So, I guess that if you delete the folder ks_zandvort from the server installation, and then you do an integrity check on your server installation, all the drivers that yesterday managed to join the race would not be able to join the server again, and will receive the same message.