Puzzles can fail to display properly

When clicking the Next button in a Puzzle, sometimes the page stays blank for a few seconds. I’m guessing that a slow database lookup is happening, yet, strangely, no “spinner” animation is shown. Either the page stays blank, or a large modal popup containing a red circle containing a red X is shown. There is no explanation inside the modal box. Sometimes refreshing the page at this point shows the puzzle, sometimes not. Sometimes it seems necessary to manually increment the puzzle number in the browser address bar to skip the missing puzzle. There can be more than one missing puzzle in a row, then the puzzle sequence resumes as normal. This happens with several of the puzzle sets.