I managed to clean up after Strongsync as follows: Remove ~/Library/Application Support/FileProvider/com.expandrive.strongsync.fileprovider (this is where the configuration files that tell macOS to mount its providers go - Strongsync doesn't properly clean these up when you delete the connection). Uninstall Strongsync (unless you intend to re-establish some connections; if you don't uninstall it, the folder above will re-appear, albeit harmlessly with just some default entries). Restart your Mac (must be a restart, not a logout/in, or macOS won't let go enough for you to do the next step). Go to ~/Library/CloudStorage in the Finder. Delete the folders that start with Strongsync (you can select and command-delete or drag them to the trash, whichever you prefer). Finder will prompt you for authentication; provide it. Strongsync should now stop automatically restarting, and its directories shouldn't appear in CloudStorage any more. Note that this completely removes all connections that Strongsync has established (which is usually what you'll want to do since unfortunately, as you mentioned, the connections aren't too reliable). If you want to only remove some, the only way I know of is to clear them all out, then run Strongsync and re-add any that you want. You might be able to examine the files in ~/Library/Application Support/FileProvider/com.expandrive.strongsync.fileprovider and edit them more strategically, but I didn't try as I was trying to clear them all out.

There are a few possible causes of / solutions to this error, some of which are detailed in previous responses. To hopefully help people coming across this as I did, I'll aggregate the solutions and add the one that worked in my case (adding .plist to the filename). For sake of example, let's say we're adding a launch agent for the user "johnny" whose service name is "com.appleseed.runthings". Make sure your filename ends in .plist, e.g. /Users/johnny/Library/LaunchAgents/com.appleseed.runthings.plist Check the syntax of the file: plutil /Users/johnny/Library/LaunchAgents/com.appleseed.runthings.plist Unload and reload your service: launchctl unload /Users/johnny/Library/LaunchAgents/com.appleseed.runthings.plist launchctl load /Users/johnny/Library/LaunchAgents/com.appleseed.runthings.plist Force load in case your job has been disabled: launchctl load -w /Users/johnny/Library/LaunchAgents/com.appleseed.runthings.plist