With the upgrade to Sabre/VObject 3 for iCal parsing/export in T48, roundcubemail-plugins-kolab now depend on a library which is not part of the source tree anymore. Therefore roundcubemail as a user of roundcubemail-plugins-kolab requires a composer autoloader configured to load classes from /usr/share/php/ or wherever the php-sabredav-Sabre_VObject package is installed.

Thus the Roundcube package also needs a local composer autoloader installed in vendor/. See iRony's composer-dist.json for an example composer configuration.