Premature end of script headers / Internal Server Error (500) / Segmentation fault / Infinite loop

Internal Server Error (500)
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, [no address given] and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Apache/2.2.16 (Debian) Server

Hi @ all,

first of all thanks a lot for these great and very very helpful modules. I'm using them a lot!
Yesterday we ran into a mysterious problem when
Node access node reference
AND
Node access user reference (Disabling Node access user reference did not help, so I'm not sure if it is truely connected to the problem, but it might be)
were enabled!

For logged in users the website worked correctly and also the permissions worked correctly for them.
Anyway the whole website crashed for anonymous users for ANY request! Not only the pages that define permissions via this module crashed, but absolutely NO page was accessible (not even the login!).

After checking logs we found out the following:
Apache Error Log: Premature end of script headers
PHP Error Log: No entry
Running any drush command on the site: "Segmentation fault"
All this made debugging damn hard, because there was no real hint what to look for.

Some other facts:
- update.php was run
- we're using the latest version of both modules
- access permissions were rebuilt (several times)
- cache was cleared (drush cc all)

This made it really hard to debug, but finally i remembered that this module was one of the last I installed and WHAO! disabling it made the website re-appear.

So there are some possible reasons for me and it would be great if you could help me to debug it and find out the reason for this KILLER problem.

Possible reasons from my point of view:
- An infinite loop in the module, possibly due to an incompatibility with another module? (Or is there any trick in configuration)
- A bug in my PHP version matching something implemented in this module
- An other strange reason?

Webserver: Apache/2.2.16 (Debian) Server
PHP Version: 5.3.3-7+squeeze19

Has anyone of you already had the same problem and has an idea where it might come from?