Thank you for your answer!
1. No, I don't have two different usernames or emails for the vaults. They are only named different.
2. I cannot connect with the second NextCloud account because
3. I cannot connect with the 2nd account because Enpass stays logged in into the 1st account and there is no error message. This behaviour is different from the desktop app.
To me it looks important to know that on my Macs I can connect with both accounts, but not in the iPhone and the iPad.
Currently I'm not able to send you a screen shot because I'm not at home or in my office this week. Anyway, there is no error message.
Let me know if you need more information.