It is an expected bahaviour.
Since ACS5.3 patch 7+ and 5.4 patch 1+ if you hit F5 (refresh) the GUI will display “ACS: Resource not found or internal server error”. You need to log out and log in back to fix that.
The reason for that is a GUI vulnerability found in ACS 5.3 and the fix for that was to disallow refresh/reload/back on the GUI. It is the same as in online bank account where you are not allowed to do ‘refresh’.
Seems like on 5.4 patch 3 they even added the pop-up explaining what you did wrong.
Just click on a specific url/menu and avoid using F5 (refresh).
Do you also see this pop-up apart from pressing F5?
What pawel explained you, is a known issue.
CSCuf93782 ACS 5.4 F5 or Refresh causes window freeze
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