This can happen when the vendor of your NAS or server tested support for the Finder, but not for the standard macOS file copy function. This function is the base of file copy operations for many apps, including SyncTime, and in order to allow these apps to copy files to/from a volume, the vendor should support it as well.

Since what the Finder does is a secret only known to Apple, it’s not possible for third-party app developers to perfectly mimic its behaviour.

Please contact me first and show me the error you get, so that I can assess whether it’s something that your hardware vendor should probably fix. If this is the case, you can proceed to ask your hardware vendor for support:

1. Download this zip archive and unpack it. It contains an app named copyfile and another zip archive named copyfile-source.
2. Run the copyfile app. The app presents two open panels: in the first one, select a file or directory to be copied; in the second one, select the destination directory where the source file should be copied to.
3. If an error occurs during the copy operation, you can proceed with the next step. If no error occurs and it looks like the source file was copied correctly to the destination, it means that the issue is somewhere else. In any case, please let me know the outcome.
4. Contact your hardware vendor (I may do this myself if they have a public email address or a website contact form that doesn’t require registering an account first). Ask them to add support for the standard copyfile function used across Apple products. Also attach the copyfile-source zip archive, which contains the source code that allows them to reproduce the issue. You can also tell them that they can contact me directly should they have any questions, either by linking to my website or my email address.