The issue is that the default.pro5pl file that contains all of your playlist information has gotten damaged somewhere, somehow, one at least one computer. After syncing, that issue was potentially pushed to each computer depending on what's going on.
To fix this, you will need to do the following steps on each computer that is having trouble syncing.
1. Open ProPresenter Preferences.
2. Click on the 'General' tab.
3. Click on the magnifying glass next to the file path for 'Application Data'.
4. Close ProPresenter.
5. Open the 'ProPresenter 5' folder.
6. Open the 'Playlists' folder. If you are on a PC, open the 'PlaylistData' folder.
7. Remove 'default.pro5pl'. If you have a custom library name, delete the file that uses that library's name.
In some circumstances, this is enough to fix the problem. You could try syncing your Library and Playlist Data again to see if the problem is resolved. If the issue continues, or exists on multiple computers, you'll need to repeat those steps and delete the playlist data file from each computer. You will also need to purge the playlist file from your Cloud account online.
1. Go to cloud.propresenter.com and sign in
2. Click on the 'App Data' tab
3. Locate the playlists data file. The file name will be a series of numbers here rather than a specific name. Click on the 'Purge' button to remove the file.
After doing these steps, you should be able to create new playlists on any of your computers and sync them together again.