RoundCube email hangs on “Sending message…”

While configuring one of my clients RoundCube installation in combination with Plesk, it would work almost OK, except that when sending mails RoundCube would hang at the spinner with “Sending message…”.
After some digging it turns out that the Plesk-bundled PEAR module directory was before the RoundCube bundled PEAR modules directory. Both contain Mail_Mime, but the Plesk version was older and did not contain the getMessage() function.
Something similar to this will pop up in apache errorlog:
PHP Fatal error: Call to undefined method rcube_mail_mime::getMessage() in /usr/share/psa-horde/program/steps/mail/ on line 492

This was fixed by changing the include_path so that the RoundCube modules come first for this specific vhost.

Don’t know what else will break further up the chain however ;-)