Search results... Missing pages? Or number reported incorrectly?

I was moving through a number of pages returned by a search -- on page 4 of 21 -- and when I attempted to move to page "5", I got a message saying, essentially, "No results. Please enter something to search for."

Is this a known problem? Or is search having troubles lately? I still get error messages a lot of the time when I search (where selecting the "refresh" option takes me back to the search dialog where re-entering the search terms is necessary)) and earlier today error pages ("504" errors if memory serves) were coming back after long delays.