1
If you read a PM from your inbox eg here on xoops.org (/modules/pm/readpmsg.php?msg_id=123&start=0&total_messages=12&op=in), you see the message and some buttons (Reply, delete, move, forward).
There are no input fields, but the form is defined as
<form name="pmform" id="pmform" action="readpmsg.php" method="post" onsubmit="return xoopsFormValidate_pmform();" >
The form tries to access on submit the xoopsFormValidate_pmform function, which is not present and not needed.
This can maybe the reason, why some people can not delete their PM,s (altough it did for me).