This bug has been around for ages, but randomly when in the lobby and the countdown starts then you are booted back to the main menu, often with an error message. Here is a weird example that happened just now, which popped up 4 ‘unexpected error’ warnings (with the trinket popup after the first one):
Here is another - countdown completes and then CTMM. Note the flash back to the desktop as well which has been happening the last few patches: