I just noticed that Usermin can't open a mail that is 6.8Mb in size. It looks like it comes with a large attachment and not inline images.
When click that message to open it Usermin will stay on 'Connecting' forever. Then, I guess related, ClamAV died and I had to restart it, or I tried to, but failed.
There were several view_mail.cgi processes that take lots of memory in /usr/libexec/usermin/mailbox/view_mail.cgi (no problem to start ClamAV after killing them though)
Reporting the large message as SPAM was no problem. Downloading mail (POP) was no problem either.
Just wondering why that happens? Would the same happen with Roundcube?