Hi @Heilpraktikerin Schulze-Pfau20 are you trying to import subscribers?
If yes, you can check this guide: How to Import a Subscriber List19
You will see an option to import a single subscriber or a CSV list. You can also try to subscribe those addresses.
If same issue occurs try a different browser or a private browser to avoid caching.
I hope it helps. If you're still having issues you can always reach out to firstname.lastname@example.org.