Upgraded to for Factorio v2.1.9, but loading current version of the mod causes the following error:

Failed to load mods: Error in assignID: recipe with name 'FastLongInserters_fast-inserter-recycling' does not exist. It was removed by kry-inserters.

At ROOT.technology.recycling.effects[239].recipe

Source: recycling (technology).
Mods to be disabled:
kry-inserters (5.2.1)
recycler (2.1.9)
space-age (2.1.9)

Looks like this mod is incompatible with space-age v2.1.9 and recycler v2.1.9. I have to turn off Quality, Recycler and Space-Age in order to get it load. I have not tested it in game yet.

I was really hoping I wouldn't have to fix this for any other mods, but it's an issue of compatibility with other inserter mods (specifically FastLongInserters or AdvancedLongInserters, whichever one you're using). I was hoping it was just a Rubia issue, but seems I needed to expand the fix to the rest of the inserters that had manual compatibility patches.

...wait a minute, how are you triggering that error? Neither FastLong nor AdvancedLong has updated to 2.1; it shouldn't be possible to hit that error, unless you have an unofficially updated version?

Because technically speaking, More Long Inserters is perfectly compatible with just Space Age. It's the compatibility with other inserter mods that is currently broken, but only when those inserters are present.

EDIT: Should be fixed in 5.2.2, for either FastLong or AdvancedLong, as well as any other inserter mods (though I am unable to test, as there are no official 2.1-compatible versions of said mods)