I have deactivated all plugins except WPML, Staging and Download Manager and the problem persists.
So it does not seem to work with WPML?
Can you try to figure out the problem? Else we need to continue our search for a new download manager plugin which supports WPML.
We can not use any other translation plugin. Most of the others did not meet our requirements and we have already put months of work into the translations with WPML.