error message on OL2010 "problem with proxy Certificate (80000000)"

Hi

I have a problem since a couple of month with following message that some Outlook 2010 Users have occasionally:
There is a problem with the proxy server's security certificate, <CAS-Array>. Outlook is unable to connect to this server. (80000000)
If i checked the connections if the message come up. Outlook works fine and all connections are ok. I have tried to enable Outlook Logging but i found no hint about the message.
I checked the KB: https://support.microsoft.com/en-us/kb/923575
Our enviroment is a 2013 CU11 4-Node Multirole DAG. All clients (only Outlook 2010) are patched with the last Updates. We have no PF since we migrated from 2010 last year. The Cas-Array was load-balenced about a F5 V11. Our Certificates comes from a internal Enterprise CA.

I also enabled Outlook logging but i did't found something.

The big Problem for me is that the Message comes very rarly.

Thanks for any assistance!

Regards,
Florian