Sorry, login is currently broken on the beta - it should be fixed on the TF tomorrow.
To work around this, you can either:
- Open the login page, and wait a couple minutes after entering your username and password before pressing "submit". Or,
- Go into the TestFlight app and download the older
2.2 (348)
version of Mlem. Once you login to your accounts using that version, you can re-download the latest version and your accounts should be kept.
Apologies for the inconvenience.