This has been solved in 6.1 so just wait for the final release.

In the meanwhile, don't use this sort, and if you're already crashed, just remove the <system>.sort=<digit> from the recalbox.conf file.

That only sorts the games (without even a genre label/separator), there's still a thousands-long list to go through.

Updates can bring new systems, so you'll have to add them to your backup