I am having similar issues as cajunqueen12 wrote in the "Problems after update to 12.3.1" thread. However, since the conversation went to private email, I'm not sure of the resolution.
I upgraded my iPhone 7 Plus to iOS 12.3.1. When I open the 1Password app on the phone, something must have gotten reset, because I now see the "Welcome to 1Password 7" screen (which has never happened before with previous iOS upgrades).
I then tapped on '1Password.com' under the 'Already using 1Password?' heading. The next screen shows "We found 2 accounts". This is expected since I have a personal account and business account. In each account line, the email address and 1Password URL are as expected. In either case, I click the plus sign at the the right, and it prompts me for the 'Master Password'. I entered the master password correctly, and a box pops up saying:
A problem with your sign in
Please check your Master Password or
sign in manually with details from your
Emergency Kit or a signed in device.
Now, I realize the immediate response is that I'm typing it in wrong. I can guarantee you that I'm typing it correctly. I've tried multiple times on both accounts. I'm also typing in the correct master password for each account. I can also verify that my master password works since I'm able to log in via a web browser. I have not changed my email address, master password nor any other configurations recently. The only change was an iOS upgrade.
Please help! I'm not sure what to do. I guess I can delete the 1Password app on my phone and reinstall, but I feel that's the easy way out. Plus, if there really is a bug here somewhere, I'd like to figure it out. If this ever happens when I'm on the road, it would be scary and very inconvenient.
1Password Version: 7
Extension Version: Not Provided
OS Version: 12.3.1
Sync Type: 1Password account