On 20-07-06 one of my hosts posted a -15,491 credit loss on BOINCstats.

When I looked into greater detail on the projects I'm attached to I didn't see any project which has removed any credits on or near that date. Is this a problem? Credit-wise I'm ont worried .. if credits wern't issued for the projects I'm attached to I'd still be part of everything I donate time to. It's just a curiosity how that happened.

As mentioned many times before the combined host stats may be inaccurate because they depend on the Host CPID which may never line up. I think this is the problem in your case. Perhaps your Host CPID has changed that day.