When saving e.g. Cookies, the data is not transferred, the reason for this is a setting at Strato Webhosting. Find and disable the PHP Booster option in your webhosting section. If you cannot find this setting, please contact Strato.
When saving e.g. a Cookie, the message Forbidden 403 appears. This is caused by a setting in Plesk.
In Plesk, the Web Application Firewall must be changed from Balanced to Fast. For detailed information please see here: https://docs.plesk.com/en-US/onyx/administrator-guide/server-administration/web-application-firewall-modsecurity.73383/