Translation fails with "There is no custom post with ID ..."

If the translation of a post fails, and you see an error message like this in the Logs:

[Query "create-missing-translation-customposts"] Execution with errors: 🔴 There is no custom post with ID '30967'

The issue might be that the DB contains stale/corrupted data from Polylang, including relationships to posts that don't exist anymore (that somehow Polylang did not delete).

A solution is to recreate the post from scratch:

Create a new post
Copy/paste the data
Delete the previous post

As the new post contains no stale metadata (i.e. no relationships to non-existing posts), translating it will work.

Otherwise (only if you know what you're doing: you can break things!), check in the DB where that relationship to that non-existing ID is in Polylang, and remove it.