We were having trouble adding a user to a site collection.  Every time we hit the search button or clicked the check userid button the system would just sit there and spin. I added the following section of the web.config to resolve the issue.

The error that we were getting in the trace logs is similar to below.

Exception Type: System.Web.HttpRequestValidationException  Exception Message: A potentially dangerous Request.Form value was detected from the client (ctl00$PlaceHolderDialogBodySection$ctl04$OriginalEntities="").

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