Can not move messages to spam in Roundcube

Help with Setup/Upgrade
1

MIAB v77
Roundcube Webmail 1.6.19

As far as I can tell, the only way that is supported by this RC version to mark messages as spam, is selecting them and moving them to the spam folder, however, there is an error message “Could not move the message(s).”
(An internet search for RC / spam shows toolbars that have a fire icon to do so, but there is no such icon in this RC version, as far as I can see.)

Questions:
How can I mark messages as spam in Roundcube?
And more general, how can I teach MIAB that a message is spam in any other way?

2

Moving a message to SPAM has always worked for me and still does. No issue on iOS email client or in a webbrowser.

3

Try to move a message, then immediately check sudo journalctl -u mailinabox -n 100 | less and sudo journalctl -u dovecot -n 100 | less for errors. Or look at /var/log/mail.log