Thought some of you might be interested by my own personal DNN 4 trauma.  I've been trying to work with version 4.0.3 on shared hosting (1&1) with a number of separate sites but with a constant stream of error messages like "DotNetNuke Upgrade Error" and "System.Security.SecuirtyException" that have brought me close to dumping DNN (even downloaded Magnolia - yikes).  I did however notice that internal links in a links list worked OK that went to pages that didn't work from the main navigation, the difference the link panel did not use friendly URLs. Having turned off friendly URLs I've not had a single error for quite a while.  The behaviour is consistent so if I reapply friendly URLs my sites go down.

This was a fluke fix as I've been looking at trust level issues, bad modules, dodgy installations, errors in my web.config and anything in any fourms that might do something!  This still leaves me with a problem I'd like solving though.  I like friendly URLs.

Hope this is useful to someone.  Unchecking a tickbox beats a reinstall.