I had this problem already some versions ago of 7.4, but couldn't reproduce it up to now...
It looks like if 1P7 gets upgraded, while the browser is running, after the upgrade the browser extension doesn't work anymore until the browser gets restartet. (Restarting 1P doesn't help)
Everything what is visible in the debug log is:
17852 18.601 WARN opw_app::browsers::opx:87 > unsupported OPX message
If I look more closely, there is no connection build up to the browser extension
Maybe you have better luck and can reproduce it without upgrading!
1Password Version: 7.4.755
Extension Version: 184.108.40.206
Browser for testing: Firefox 74.0
OS Version: Windows 10 1909 18363.720
Sync Type: Dropbox