What is the correct way to handle this, until the new clamav plugin, which will be not free anymore, will be released? Should we stay in this situation until then, disable the clamav plugin, or is there any fix for the moment? Unfortunately the clamav readme from the i-mscp plugin is removed, therefore I wasn't able to check and fix the config change myself.
The README.md file is part of the plugin so you should have access to it under the /var/www/imscp/gui/plugins/ClamAV directory.
After restarting the clamav-daemon, and the clamav-milter filter, what's the content of the /run/clamav directory, and the /var/spool/postfix/clamav directory ?
Also please, post the content of your /etc/clamav/clamd.conf file, and the content of your /etc/clamav/clamav-milter.conf file. I'm pretty sure that your milter configuration file is wrong but...
BTW: Don't forget to surround your copy-past with bbcode tags.