Checksum errors can stop the data sync with a red banner. Sometimes but not always this might show as " File Doesn’t match Checksum "
Usually, this is due to a mismatch in "last modified" time between the remote client and the server or MEME type. This can normally be resolved by removing the files in question and letting the sync client pull down the correct version of the file. If this fails, you can more directly bypass the issue by pausing the sync connection on the client, downloading the files in question (usually as a ZIP file) from the webGUI, placing them in the proper folder in the sync location, restarting the sync connection and allowing the sync to complete. Finally if that still does not resolve the issue, and especially if this occurs repeatedly for the same user, it may indicate that there is an issue with the user's sync client application, in that case you may consider reinstalling the sync client, or updating to the most recent sync client version.
If you are still having problems contact support.