Used version: McAfee Web Gateway 6.8.6. build 6257
Is there a simple way to find out what i need to disable when certain websites aren't show properly.
The categories of the websites are allowed.
For example: http://ezproxy.library.wur.nl/login gives end-users a white screen.
That can be difficult to answer since this is specific to your configuration.
You could start by looking at the access log to see which contents of the page were blocked and allowed. If your access.log contains the block_res value, you can decode that to determin which parts of the page are being blocked and why.
the block_res values are documented in the reference guide
Then you make changes to the config based on blocked requests.
But this assumes that page content is being blocked by a policy setting, and not failing for some other reason.