This server was also upgraded to version 6.8.28 before we saw all the problems with the e-mails, in any case on this server the problem is that creating domains or installing from one click the software or presented 403 error or SSL error. the tests that have been done are in order:
1) server restart
2) fix panel web
3) bt 16 ( ssh )
4) suspension and reactivation of domains
but nothing has changed
ps error 403 also reported by phpmyadmi and here too the tests were the same
which did not happen before the update to 6.8.28.