Up till quite recently (3.1.2?) encrypted mails couldn't be moved at all, but for a workaround where a different mail is marked and the mouse is used to move the unmarked mail.
This is still (3.1.4) the case in the Junk folder. This is a problem, because I don't receive Junk often and the encrypted content leads Outlook to mistakenly put those mails there. So typically there are only encrypted regular mail(s) in the Junk folder, and I can't move the remaining last mail at all, but for a workaround for the workaround ;)
- move a regular non-signed, non-encrypted mail to Junk
- mark it there
- move the encrypted mail by mouse
- move the regular mail back