שגיאת ייבוא גיבוי: "לא ניתן ליצור אינדקס ייחודי"

Just tried importing via the GUI, with exactly the same result:

The old server doesn’t have a file called PG_VERSION, how can I tell what version its running? I’ve updated the docket install to the latest version today.

The new server (newly bootstrapped) is running postgres V13

cat shared/standalone/postgres_data/PG_VERSION
13

Is there a recommended procedure on how to do this?