In my Server (Plesk) the problem was caused by the activation of some Mod_security (Web Application Firewall) security rules.
Mod_security is the CMS-level firewall that examines the requests made to the server and categorizes them as malicious or not.
Through Plesk's error logs, the rule is recorded as well as its ID, where the cause of the error was.
So I added a rule (ID) from the Web Application Firewall to the Security rules IDs field
ATTENTION: In case there is more than one exception, add the IDs below each other and not next to each other, no commas, no spaces.
NOTE : There is a chance that the change will appear online within 60 minutes.