Litespeed and Plesk work in most cases without issues. However, the webmail (Roundcube and/or Horde) server might require additional tweaking when you have one of the following errors (or similar to unknown PHP methods/classes):

Example 1

[11-Aug-2020 19:36:56 UTC] PHP Fatal error:  Call to undefined function filter_var() in /usr/share/psa-roundcube/program/lib/Roundcube/bootstrap.php on line 49

Example 2

[12-Aug-2020 09:09:12 UTC] PHP Fatal error:  Call to undefined function mb_internal_encoding() in /usr/share/psa-roundcube/program/lib/Roundcube/bootstrap.php on line 86

The problem

Litespeed uses the default PHP version assigned to lsphp, which usually is PHP version 5.6. Plesk, however, requires PHP version 7.3 to run Roundcube smoothly. This is also the default PHP version for webmail defined by Plesk, but Litespeed ignores this.

The solution

By linking lsphp to the correct PHP version, you can resolve the issue.

mv /usr/local/bin/lsphp /usr/local/bin/lsphp-bk
cp /opt/plesk/php/7.3/bin/lsphp /usr/local/bin/lsphp
cagefsctl --force-update
systemctl restart litespeed