I joined a server with a car I don't own, OK so far but the error came up again after a couple of minutes when I was setting up an offline race instead. Back to the main menu and continue setting up the offline race. It came up again, about a minute later. And again about a minute later. And again. Eventually I quit the game and restarted, everything was back to normal.
I just noticed that I have 12 copies of the .rfcmp file which have been downloaded from the server (up to "c:\Program Files (x86)\Steam\steamapps\common\rFactor 2\Packages\BMW_M2_CS_2020 (11).rfcmp") - I logged into in twice so the other 10 were perhaps related to my attempts to continue offline.
