Visit website for fritzbox login which is the IP of the router and by default an http URL.
When on login website and selecting the fritzbox 1P entry with correct website URL nothing happens and no login is filled.
Login should be filled and submitted on "Enter" press.
This seems to be an old problem. I found a discussion from 2014 referring to this, although I am not sure it is the identical problem:
1Password Version: 7.2.5
Extension Version: 126.96.36.199
OS Version: 10.14.3
Sync Type: Not Provided