API - WebFilterURLGroup object won't be deleted.

Hello,

I cannot remove WebFilterURLGroup that I see on the web console that it exists. 

My request for remove is according to API v17.5: 

<Remove>
<WebFilterURLGroup>
<Name>Test Group</Name>
</WebFilterURLGroup>
</Remove>

the response I get is:

<WebFilterURLGroup transactionid="">

<Status code="500">Operation could not be performed on Entity.</Status>

</WebFilterURLGroup>

</Response>

I checked the /log/apiparser.log and that's what I got: 

and when I go to the web console the object is showing and not removed the way I want to.

Please help me, it's urgent.