Plesk Obsidian is being used as a provisioning node on WHMCS 7.8.3
Through WHMCS, user password fails to be changed with the API error:
CONFIG_TEXT: [errtext] => Argument 1 passed to Checker::sys_passwd() must be of the type string, null given, called in C:\Program Files (x86)\Plesk\admin\plib\api-rpc\AgentHosting.php on line 377
Bug PPP-44614 in Plesk Obsidian
Wait till Plesk Obsidian update published at the end of October.