I had the problem that after configuring Sharepoint for .Net 3.5 suddenly I could not edit some sharepoint pages anymore. At first I changed the debug settings in web.config to get more clear error messages (instead of „an unexpected error occured“).
See here for Details for enabling clear error messges
After that I got the strange error messsage „A potentially dangerous Request.Form value was detected from the client“…after 2 hourse researching I found the solution…..
Somehow a section inside the Web.config has disappeared (or I deleted it accidentely…)