Yes, that ü looks like our current ö problem, the previous occurence of the problem was using a + sign in the file name on a windows server.
(so both bugs are occuring under 2.5.+ and I read that is also under 2.6.+ )
By the way, the file is stored under that name, but the system cannot read it's own written path...
>> Why not extend the filtering-routine that also changes the space into an undersocre in file-naming?