Plesk v11.5.29 (or higher) was an error Webmail.php.
Plesk v11.5.29 (or higher) was an error. That is the file comes from Webmail.php. I think, this has been created by a programming team of Parallels.
When I delete one customer, it has a specific error message as follows:
Internal error. ERROR: Call to a member function delete () on a non-object (Webmail.php: 106)
Request corrected soon. My email: hoahc@matbao.com - I'm buying a license Parallels with IP 112.78.2.212

We welcome suggestions here, but posting assistance requests are not much efficient here.
The problem cannot be reproduced on our side, so it may be a subject of very specific local configuration.
We can recommend to contact support service for detailed investigation or continue conversation in http://forum.parallels.com/showthread.php?288231-Plesk-11-5-30-ERROR-Call-to-a-member-function-delete()-on-a-non-object-(Webmail
Further assistance cannot be provided without more information from your side.
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Hoang Hoa commented
Hi Sergey L,
hthntts is me (sender requests from forum.parallels.com). But I still have not found satisfactory answers your.
Currently, this problem would still not correctedhthntts in link: http://forum.parallels.com/showthread.php?288231-Plesk-11-5-30-ERROR-Call-to-a-member-function-delete%28%29-on-a-non-object-%28Webmail