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I was always using the direct Add to Page Tab URL like this:
http://www.facebook.com/dialog/pagetab?app_id=238557109577754&redirect_uri=http://www.facebook.com
And today that URL gives me "Sorry, something went wrong.". Did something changed on Facebook side? Or is it a temporary bug?
Note that I have tried other redirect_uri params, such as the URL of the location where the app is hosted. It does always result in the same error.
Edit: It's a bug: https://developers.facebook.com/bugs/304168516342380
You can use Graph API Explorer to add your page tab over API as a workaround. See the link above for details.