Invalid Steam ID Ticket
From Obsidian Conflict Wiki
If you get constantly kicked with the "Disconnect: Invalid STEAM UserID Ticket" message, shut down Steam, go to your Steam folder, delete the file "ClientRegistry.blob" and start up Steam again.
Update 25.11.2011: Recently we are getting a lot of reports of people unable to join Sourcemods (including OC) due to that error even after deleting the .blob file, if that's the case for you, do the following:
Shut down Steam, head to your Steam folder and delete everything except:
Steam.exe
"Steamapps" Folder (which contains your games so you won't have to download them again.)
[Optional to delete] "Userdata" folder (which contains your screenshots and save games.)
And start up Steam again.
