Size of a request header field exceeds server limit.

Hi guys,
On one of my ISPConfig servers I continously get the error message:

To fix it locally I just have to clear the cache but I'm worried that this error also occurs for the rest of my visitors.
Does anyone have any suggestions on how to resolve this?

Edit:
This error occurs on all sites of the server i.e. when this error occurs www.site1.tld and www.site2.tld all show this error.

Edit2:
I should perhaps also add that even the ISP Config control panel show this message unless I clear cache.

 

I have tried adding:

To /etc/apache2/apache2.conf but that did not do the trick.