Some users have reported that enabling Full Disk Access for BkmxAgent as explained in this article did not result in success.  Please do that first.  If you are sure you did everything correctly,  please continue with these steps…

UPDATE 2022-11-22.  It seems that, if you have macOS 13, and possibly some earlier versions, it is no longer necessary to add BkmxAgent to Full Disk Access as described on this page.  Whoopee!  However, because Apple does not publish or announce their security measures, we are leaving this page up in case it becomes necessary again.

• Activate Finder.
• Click in the menu: Go > Utilities.
• In the window which opens, find and doubleclick to launch Activity Monitor.
• Into the Activity Monitor window which opens, into the search field at the upper right, enter BkmxAgent.  The list will collapse to show only BkmxAgent processes. There should be one, but there might be more.  Keep an eye on that window.
• Activate the subject app.
• Click in the main menu, if enabled: name of app > Stop all syncing now…; then click the button Kill.
• Note what happens in Activity Monitor.  Did one or both of the BkmxAgent disappear?
• If not, select the remaining BkmxAgent(s), then click the ⊗ (circled X) button in the toolbar.
• In the dialog which appears, click Quit.
• Wait 12 seconds.  If either or both of them reappear, click the ⊗ again, and this time Force Quit.
• Quit the subject app, affirming that yes syncing is paused (do NOT re-enable syncing.
• If you think you may have an extra copy or older version of BookMacster, Synkmark or Smarky lying around somewhere, trash it.  On a good day, you can find these by typing the app's name into a Spotlight search.
• Restart the computer.
• Launch Activity Monitor and search for BkmxAgent as before.  Verify that none are running.
• Try to grant Full Disk Access to the subject app and BkmxAgent, again.  If it still fails in the end, return here and continue with…


MORE THAN THE KITCHEN SINK

The following procedure descriibes how to add a copy of BkmxAgent into Full Disk Access, instead of the real BkmxAgent, then delete the copy.  It is very strange and not recommended unless you have tried multiple restarts.  We have seen it work in macOS 11 Big Sur.  We have no reports of this being necessary in macOS 12 Monterey.

• Activate the subject app.
• Click in the main menu, if enabled: name of app > Stop all syncing now…; then click the button Kill.
• As in the first half of the above, ensure that Syncing in our subject app is not configured or paused.
• Quit our subject app if it is running.
• Activate Finder.
• Open a window to your Documents folder. (Click in the menu: Go > Documents.
• Click in the menu: File > New Finder Window.  A new window will open and activate.
• While holding down the command (⌘) and shift (⇧) keys on your keyboard, type G.  A sheet with a small text field will appear.
• Copy and paste one of the following paths (depending on which app you have) into the small text field:

    /Applications/BookMacster.app/Contents/Library/LoginItems
    /Applications/Synkmark.app/Contents/Library/LoginItems
    /Applications/Smarky.app/Contents/Library/LoginItems

   The window contents should change to show a folder containing a single item, BkmxAgent or  BkmxAgent.app

• Holding down the option key on your keyboard, drag BkmxAgent(.app) and drop it into the other window, thereby making a copy of it in your Documents folder.  Finder should display a "+" character in a green circle as you drop it.
• Look at the System Preferences window again.
• If there is an existing BkmxAgent in the list, select it and click the [-] button below the list, to delete it.
• Close the Finder window to LoginItems.
• Drag the copied BkmxAgent(.app) from the Documents window into the Full Disk Access list.
• Ensure that the BkmxAgent appears in Full Disk Access list, and that its checkbox is ON.  (If it does not appear in the list, try to quit System Preferences, relaunch and then add BkmxAgent again.)

That should do it.  Now, test as you did previously.  Since you know what you are doing by now, we'll be more brief this time…

• Launch our subject app.
• Switch on Syncing.
• Click in the menu: BSoS > Reboot Sync Agent.
• If it responds Reboot succeeded and no error is indicated, the problem is fixed.

• Once it is working, you may delete or trash the copy of BkmxAgent in your Documents folder.