Hi, I hope you are doing Ok.
The error is cause by a mismatch of versions in the synergy file.
In order to enable those options the synergy files must have the same "year" so the application is recognized.
I have Max 2019.1 and Mudbox 2018.2
I manage to solve this issue (Kinda* see note below) by modifying the synergy file this way:
NOTE: Backup this file prior to modifications (Just to be safe)
Path: C:\ProgramData\Autodesk\Synerg\Mudbox.2018.2.Windows.64.syncfg
(The folder is hidden if you cant see it)
Now simply change the String Version to "Mudbox 2019", and Numeric version to "2019". Leave the current Executable path alone. (This is a screenshot of my file)

Save the file.
(I dont remember if you need Admin permission to modify it), if you do, simply type cmd in the search bar of windows, and when the black icon appear, click right button od your mouse and select "run as an administrator". Then launch a notepad.exe and open the file from there.
Now with this "send to mudbox" should be accesible now.
Please let me know if you can use it without problems.
Note from above:
However In my case I got a weird behavior sending from max.
If I try to send from max to mud, I got an error about invalid paths in mudbox.
Or sometimes I send to mudbox, and mudbox dosest do anything.
In case you see this behavior:
To solve this issue, you must open mudbox, before sending from max.
Once mudbox is open, create a box or simple mesh, and send it to your scene in Max First.
Then you get a green message in max that says "Connected to mudbox"
After this is done, now you can get your meshes back and forth without problem.
Please let me know how you do.
Have a great Day.