Don't have a clue but here are a few things you can try:

Look at the file name and make sure it is in the right format and doesn't have an illegal character.

(I save my files last, first name & fist name.  System let's me do this and works just fine except when it doesn't.  Some years later I learned that the & is an illegal character)  Point being assuming all characters are legal is not necessarily the case.

Look at the size of the file to make sure there is data in it.

If you are restoring the file and there is already a file in your data base with that name move the existing file out of the folder before you restore the file.

Turn everything off and restart to reset everything in ProSeries and with the machine and try again.  If you are using a network that may mean powering down the network as well.

Let us know if anything works.