Members can't sign in, "Please enter a valid email address"

My issue is that some members superlevel.de can’ sign in. The problem only affects some users who get the “Please enter a valid email address!” error message. They also don’t receive the login mail.

The site is running Ghost Version 3.37.1 on a self hosted Ubuntu 20.04.1 LTS server by Hetzner with a MySQL database (Ver 8.0.21). Node is v12.18.3.

The theme is a slightly customized version of Lyra. Paid members are activated, free members are deactivated. We’re managing payments via Steady (a service similar to Patreon) and manuelly enter them with name and mail address.

Some of our first members reported that they received “Please enter a valid email address!” errors. Their addresses are set up in the members area in the backend. After the latest update introducing the portal the error is still there, but saying “Failed to log in, please try again”.

I can reproduce the error myself when I enter their mail adresses in multiple browsers. We tested the feature with @gmail.com and @posteo.de mail addresses without issues, that worked then and still works now.

Curiously, entering completely random mail addresses like “asd@asdasdasd.asd” seems to produce a success message. So far I’ve only seen the error with addresses who are members in the backend.

I made some changes to the signup.hbs0 file, mostly translation, redirecting the “Signup” button to our external payment provider, but nothing huge. Is it possible I might’ve broken something here?

Since fake mail addresses work, I suspect that it has something to do with however the address is verified. Where can I check these settings? Are there logs that might help with figuring out the issue?