After installing 1Password X it started showing this pop up each time I have opened the browser and unlocked it.
The pop up was about it found an account and want to add it to my mac, I was already signed in to the desktop app for ages now though.
Providing a master password after clicking on "Add" button did act like it was an invalid one every time.
I had to go to accounts in my desktop app and remove my account from the machine, then the pop up Add button did work and it re-added my account to the desktop app.
Quite annoying that I had to remove my account first and re-add using a browser extension. I know it's beta feature, but might worth a little debugging and maybe do those steps automatically or at least give an explanation to integrate it properly it needs to be done - I couldn't figure out why my password didn't work, it was lucky guess it might have trouble with already added actually account. I have also sent an email as I wasn't sure for a moment if it's not a phishing pop-up from something else than 1Password X.
1Password Version: 7.3.BETA-4
Extension Version: 1.15.2
OS Version: OS X 10.14.4
Sync Type: membership
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