Migration to PokePC doesn't work (Failed to migrate classic living dex)

Reported by Bluenote (discord):

Hey guys! I think i need some help with the migration. In the past i've used the "National: Grouped by Regions (Recommended)" on suppereffective.gg. I created a new account on PokePC and linked it to my supereffective account. However, the migration doesn't work and i don't know why. Is this still in progress or is the error message caused by something else?