Amazon Cloud drive sync stops while incomplete and switches to live synchronisation

Hi there, I am having a problem here with the Amazon Cloud drive sync using a ReadyNAS 104 with firmware 6.6.1

At the beginning all went smooth when I started synchronisation with 3.5 TB of data to upload. At 10 Mbit/s upstream it took several weeks. Now with more than 3 TB uploaded, the service is more and more often failing with this kind of error message in the protocol:

"Failed to upload /data/Share/Folder/File.xyz. Please check the file path name. Amazon Cloud Drive is case insensitive. Change the file path name and try again.."

Then the uploading stops and it usually says: Scanning of local drive is completed / First check for changes done, live sychronisation starting. / Amazon Cloud Drive service started.

But the files are still incomplete, because upload has stopped at the failed file.

When I then click the "synchronize" button at the configuration for the Amazon Cloud app, it will scan the drive again, rebuild the index, check for changes and then resume upload.

What is the problem here?

I checked if there is anything special about the files – on Linux systems there can be a "File1" and "file1" in the same directory. But we only use Windows PCs to save the files and there is nothing like this.

However, at the failed files, I always find similar filenames. Like:

File_1.prproj

File_1.wmv

File_1.avi

File_2.avi
File_3.avi

etc. – But that could be a coincidence. All failed files were big video files (1-6 GB).