I’m experiencing the same thing on about 15 different sites (no theme in common, not even all on the same server). You’re right that replacing l10n.php seems to solve it … even when the file is otherwise exactly identical.
It’s not the most elegant solution, but since keeping that file fresh is apparently essential to holding off the 500 error, one thing I’ve found is to keep that file constantly refreshing via a cron job or watch statement.
But since this isn’t a hack and doesn’t seem to be a one of a kind bug at all, it should really be brought to WordPress’ attention for a potential fix in 5.5.2 or 5.6!
