I was getting this UrlRewrite exception a lot on my development machine (WinXP, XP3200+, 1Gb DDR400, VS2005) but never on my staging server (Win Server 2003, P4 2.66, 1Gb). It seemed to happen often if I opened a 2nd browser to localhost, and especially if I was a bit quick between the build of the site and the browser refresh.

Now the interesting thing... I've upgraded my dev machine to 1.5Gb of RAM and I haven't seen the problem since. I got the upgrade a couple of days ago as I was pushing 1Gb in use at times (VS2005, SqlStudio, Dreamweaver, IE, FF, Opera + other bits and bobs open).

Another thing I noticed is that when the problem did occur a browser refresh was next to useless, it would work maybe one time out of 20, but an iisreset always fixed it.

  I've took a look at support.dotnetnuke.com and found many of bugs causing the nullreferenceexception to be thrown have been found and fixed as well. But as far as I don't know what is the realease date of the next stabilization release of the 4.xx I've fixed them myself. Including the following:
1) NullReferenceException thrown at random due to pattern used to work with cached objects in core modules.
2) The issue in the Localization.vb at line 192 (at least as I remember).
3) The issue with edit permissions assigned to roles.

  If somebody wants I can send the compiled version (rared bin directory). Replace the one located in the bin dir and that seems going to solve your problems. However I can guarantee nothing.
  Email to: amirit@front.ru. The size of the package 0.9MB...

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Best regards, Evgeny

I've forgotten :) The source files could be sent too :)

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Best regards, Evgeny