According to erdnaxeli in the discussion starting at https://matrix.to/#/!cURbafjkfsMDVwdRDQ:matrix.org/$1499518666138684TsxJg:cervoi.se, failing to join a room caused a popup to display a CORS error. @ara4n had previously fixed all the error popups to be more friendly, but either he missed one, or a new one crept in.

Got the same error right now on /develop (room #matrix:matrix.org)
My error is HTTP error 504
Dunno what the underlying error is, but we should never be vomiting up that kind of error message direct to the user :
Apparently, we also get the CORS error popup when it fails to change settings, as per https://matrix.to/#/!cURbafjkfsMDVwdRDQ:matrix.org/$1505422396418vKhhA:matrix.freelock.com
FWIW I haven't seen any token, within an error dialogue, recently.
Closing in favour of https://github.com/vector-im/riot-web/issues/7785