Go to the inbox, mark the message in the radio box. Select "Delete marked". That action gives me

"The submitted form was invalid. Try submitting again"

Looks like a bug to me and not a feature. I tried it on a few updated boards that all work perfectly, they had the same issue. All boards tested are on different operating systems, all using different versions of php (7.2,7.3,7.4).

Topic:

https://www.phpbb.com/community/viewtopic.php?f=661&t=2535026