I made some tests and the problem only appear if i work remote in the ePO. When I log into the mashine where the ePO is installed an open it there I can import the file. So it seems to be some kind of problem with the rights.
I had the same problem importing a list of device serial numbers which I solved by ensuring the first row of the csv is alpha-character only (a column name such as : 'value'). But ePO will import that 'row' along with the other values, so make sure you delete it afterwards. Any numerics in the first row will result in the error message: The file was empty or has data in an incorrect format.
Correct csv content example: