Been using Thunderbird for years without major problems. For a few days now the 'Delete' button in the top section of the e-mail doesn't work all the time. For some mails the button works, other times it doesn't. Instead of deleting the message, this menu pops up:

I did try removing the Trash files, but that's not the issue. Also, deleting by using the menu from right-clicking works, both from the message body and the email topics list, so it looks like the probelm is in that button. Also 'Delete' button in the keyboard works normally.