[STDERR] PHP Warning: Base directory /usr/local/CyberCP/public/mailwatch/mailscanner/lib/htmlpurifier/standalone/HTMLPurifier/DefinitionCache/Serializer does not exist,
please create or change using %Cache.SerializerPath in /usr/local/CyberCP/public/mailwatch/mailscanner/lib/htmlpurifier/HTMLPurifier.standalone.php on line 15754
No function is missing bro. Its already a reported issue with mailscanner html purifier as it doesnt create the directory by default. If you google it, you will find this issue reported in mailscanner.
But its just a warning which you can ignore or simply create that folder to get rid of this notice.