I searched some weeks for this.
cause:
Meta-Data for subscribed catalog has an error.
solution:
Delete the corrupt/wrong file(s) under \WSUS Package Publisher\Subscribed Catalogs\MetaData
I had only one file there from a first try to add a custom catalog.
Auf deutsch: Die kaputte Datei(en) im Verzeichnis \WSUS Package Publisher\Subscribed Catalogs\MetaData löschen.
cause:
Meta-Data for subscribed catalog has an error.
solution:
Delete the corrupt/wrong file(s) under \WSUS Package Publisher\Subscribed Catalogs\MetaData
I had only one file there from a first try to add a custom catalog.
Auf deutsch: Die kaputte Datei(en) im Verzeichnis \WSUS Package Publisher\Subscribed Catalogs\MetaData löschen.