I got 15 of them during the evening while my screen saver was up. All kinds of graphical glitches were going on while I was not around to see the reports. However I dont know how to determine what the error is, and would rather figure it out then post the entire thing here.

Look for the one that says that it's the first.

right, but what in the file is important since there are lots and lots of raw data.

I am no developer, but as an end user I find that the most valuable piece of information in a crash report is the line about a page down that says "Crash Location". Foobar itself is rather stable and tha majority of crashes one encounters are caused by 3rd party components. This line typically will tell you which third party component is causing the crash, and then you can visit the thread for that component to check for updates or report a bug. Even if the crash is not caused by a third party component this is a good thing to check before pestering the main foobar developers with possible bugs. Also if you foobar is crashing repeatedly, is the "Crash location" the same or does it change from one report to another?

Sometimes your diagnosis is right when you say "foo_xy.dll is the problem", but the crashlog says "crash location: ntdll". How do you know it then? Is there some methodology or just knowledge about the plugins bugs?

generally, crash location is useless.
you should check the stack dump location, from the top to bottom, and most likely the first location that states foo_xxx_xxx.dll is the one you are looking for.

perhaps we can put together a wiki page that is a list of things to look for in crash reports. I think it is silly to post those huge things in the forum everytime we have a problem - it could be much more useful to know what to look for otherwise.