User webinterface claims file does not exist

Uploading through the webinterface is ok, up & downloading through the Apps is allso fine.
Only downloading the file through the webinterface gives me an the error “file does not exist”

I thought it may be the problem with the spaces in the filename, but this is not hte case (it allso accurs in filenames without spaces).

How can i debug the cause of this behaviour, or even better, what can be a solution for this problem ?

p.s. i allready checked if system admin/settings/url settings for FILE_SERVER_ROOT & SERVICE_URL are correct.
And they are set correctly.

Kind Regards