When composing a message to more than one user, we get an immediate error message that says “There has been a critical error on your website”. The message will still be sent and you can find it in your sent folder. However, it does not send the copy to the user’s personal email. Sending a message to one single user works fine (with no error message and a copy sent to the user’s personal email).
Everything was working fine until we updated to BuddyPress 6.2.0 and WordPress 5.5. We also run a highend theme.