no posibiliy for correct upload files

Hello everybody.

We having here a big mystery, perhaps anybody have an idea..
if we wan't to upload some files we must try more than 3 times.
Then suddenly it works. But if the next user want to download the file,
the files comes incorrectly to space. Some of them changes even the original extension.
An XLS-File become the name: XL
An PDF become the name PD.
Thank you for some ideas or help!

regards
Catharina