Hey, Timo, I had the same issue and it seems the app does not recognize all valid IBAN number formats.
I tried a random German IBAN sample from the internet and it works fine. But when I use my real Latvian IBAN, it isn't recognized and I get a "missing IBAN error".
My actual one (unrecognized) is in a similar format to LV99PARX9999999999999 or LV99HABA9999999999999 (numbers redacted, of course).