SP2010/Wiki Pages: "Sys.WebForms.PageRequestManagerServerErrorException: An unexpected error occured." when editing a page

Over the last few weeks I came across one problem multiple times. When you tried to edit a Wiki Page in SharePoint 2010 you got the error Sys.WebForms.PageRequestManagerServerErrorException: An unexpected error occured. This was quite wired, as the users had “full contol” of the wiki. So, first we look at the ULS logs at the…

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