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I think I found the problem (which should NOT happen, I am really puzzled).
The behaviour you described only happens to me when the file size of the file (bytes) is higher than the max. upload size (which is set in debaser preferences). The theory!
says that there should be a failure warning if this happens, but this is not the case. Please compare the upload_max_size-value in your php.ini and the value you set for max. upload size and maybe set it to a higher value.
This bug will be fixed in the next version.
To the "already exist" problem: Erm, cough! My fault, I never expected to develop this module so far. So there are still remains of the only-mp3-player-past inside the code. What happens is a comparison of the title of the file. If there are files with the same title there comes an "alread exist" warning.
If there is no title (especially for non-mp3-files) the file name will be taken as title, but if the upload fails due to the bug I described before, there is even no file name and if you try it again with another file and the bug occurs again then there are two database entries with the title 'blank' which is forbidden.
I hope you'll find my explanations understandable and helpful.