Same issue happening for me. Dozens of my users are reporting the same bug. They randomly got signed out and even though they try to use a correct email, the app says the email is invalid but will still send a pin.
It appears to be a per-device issue that then applies to all apps accessed on that device.
While working on an app today, before anything got weird, I cleared my browser cache a couple of times to try logging in with different email accounts - I hadn’t yet realized how to add the menu-based logout button.
At some point I started getting the same error as OP described.
If I try logging in on a different phone or browser it work fine.
If I try any email address on any (?) app on my “bad state” phone, it says “not allowed”. To test that I made a new app with a brand new spreadsheet backing it and immediately encountered the same issue.
My apps are set to “Public with email” privacy.
And like OP, it does send me a PIN each time before giving me the error message.
The device having trouble is an iPhone X. I’ve tried clearing Safari web site date, closing all tabs, killing Safari, and rebooting the phone. Nothing helps.
In terms of the per-device piece of this. I tried my newly created app on both my wife’s phone as well as my own and both were getting the same error that @Rosewebstudio provided regardless of the device. I then tried to copy the app, including duplicating the page entirely and I was still seeing the same issue
Yes, I got the error when trying to favorite a “talk” and when trying to create an account through the sign in on the menu I was not able to progress past inputting name and email. In both cases I was getting pins emailed every time
When prompted by the favorite feature I saw the same error in the screenshot provided by @Rosewebstudio.
When trying to Sign In from the menu I would not advance, it was as if I had not pressed submit however the email disappeared, my name remained and I received a pin without the page moving forward.