I have made a fresh install of Roundcube and everything seems to work, but I get some warnings in errors.log. This one is logged every time I send a new mail in Roundcube: [25-Aug-2021 16:29:15 Europe ...

Hello, Version 1.5.x requires PHP 5.4-7.4 as per README.md Downgrade to PHP 7.4 or use the 'master branch' for PHP 8.x.