Three users have reported seeing this error recently. I think it might happen after an upgrade to version 3 of our apps, if the BkmxAgent from version 2, which does not play well with Safari 16, got itself stuck.
There are, I think two alternatives to fix this:
Alternative #1Log out and back in from your Mac, or Restart the computer.
Alternative #2Summary: Quit both BkmxAgent processes (since we don't know which one is bad), and restart syncing. Here are the steps in detail:
• Activate our app.
• If
Syncing is Ready, click the button to make it make it
Paused…

• Activate Finder.
• Click in the menu: Go > Utilities.
• Launch the application named
Activity Monitor. A window will open.
• In the top right corner of the window is a text fieod for filtering results. Type
BkmxA into that field. The list should collapse to show only any
BkmxAgent processes that are running.
• Select one and click the "X" tool in the toolbar.
• Confirm that you would like to "Quit" the process, and if that does not make it disappear, "Force Quit".
• Repeat for any other running
BkmxAgent process.
• Activate our app.
• If there are changes from browsers you want to import, click in the menu:
File >
Import from >
whatever• Click the
Syncing button again so it is
Ready.
• Verify that one and only one new
BkmxAgent appears in Activity Monitor.
• Click in the menu:
App-Name > Reboot Sync Agent.
UPDATE 2022-10-17: Starting with version 3.0.8 of BookMacster, Synkmark and Smarky, available in beta soon, this issue should be gone. If it ever discovers more than one BkmxAgent running, instead of displaying an error, the app will perform the above steps without bothering you.