Desktop can't sync files with ++ or xxx.config in file names

On several occations files with names containing “++” or ending with .config will not sync correctly thru the Windows Desktop Client. When looking thru the web interface, the files are visible, but are not readable on the server. The Desktop Client says there is a 404 error when accessing the file.

The problem is not consistent. It does not happen on all ++ or config files, and there is no logic on which files that fail. All files have been originally synced from one of my machines, but are unsyncable from other machines. It is not consistent which machines that fail. All run Windows Desktop Client 3.4.0.

Anyone with similar problems?