Note:- Do take Backup of site collection before following this procedure
Admins working on SharePoint might have encountered this error ( it was reported to us only 2 times in 3 years)
The server block is not well formed
You will find lot of articles on the internet stating to use SharePoint designer to reset definition of the page which is throwing this error.
This can be achieved without designer as well by going into site actions --> site settings of Site collection.
Click on Reset Site Definitions:
Enter the url on which you are getting the error
Click Reset to Fix the problematic Doc library /form .