Trying to solve this problem you would normally think that you want more details about the error. in order to get more details you would need to change this custom error page and see the full call stack. in order to do this you would change web.config CustomErrors settings and the call stack settings. the web.config is the web.config of your default website. so go ahead and change these two settings as follows.
CallStack="true"
customErrors mode="Off"
customErrors mode="Off"
After you change these two settings and try to add a column to your list in sharepoint or modify an existing wikipage sharepoint will tell you more details about the error and you know what the error was? it was in validateRequest=true setting. Sharepoint does not like this setting to be true. so go ahead and change it to false as follows
validateRequest="false"
this will basically tell sharepoint to not validate the request for inclusion of javascript which will make your sharepoint subject to XSS attacks but it will solve your problem of not being able to modify existing sharepoint webparts like wikipedia and list columns. Weired but this is how to solve it.
