I know it's not the first thing I'd try due to the work involved, but you might try rebuilding this layout.
Create a new blank layout.
Copy all layout objects except the web viewer in the old layout to the clipboard and paste them into the new layout.
Recreate the web viewer without copying it from the original layout.
Test the rebuilt layout and see if it has the same problem.
You might also test a recovered copy of the file. I've now seen two different instances where I recovered a file and FMP reported "no problems found" yet performed correctly when the original did not--I'm guessing that the recover operation set some data structure in the file back to "factory specs" and that corrected the problem but then didn't log this in the recover log.
I wouldn't then use the recovered file, but at least I'd know what caused the problem and could either revert to an older back up copy or rebuild the file.
Thanks, Phil. Rebuilding the layout made no difference. But fortunately my latest backup was missing only that splash layout with the Web viewer. A fresh layout built in the backup now works fine.
Very strange. I guess the moral is: don't ever "break" something on purpose for a shortcut.
Perhaps a better moral:
Always make frequent backups of every file.