"Can't send message" error when replying to specific contact


We’ve been using the Rainloop app since June and everything was working fine, up until the last few weeks, when two users have started getting the error that the message can’t be sent (or saved as a draft) when replying to one specific user. The nextcloud log or the server log don’t show any errors, so I’m at a loss of what to do… Did anyone maybe have a similar problem and has successfully solved it?