Cannot write tags on some files

I have mp3s stored on a freenas server. A few, files, always longer tracks, I get error when trying to change the tag.
An example:
Two error boxes appear, first;
"There is not enough space left on device \\Freenas\120GB. Please ensure at least 15,53 MB drive space."

There is plenty of room on the server..

Second error says:
"File "\\Freenas\120GB\musikk\Iron Maiden - 1995 - The X Factor\01 - Sign of the Cross.mp3" cannot be opened for writing."

The file is not in use...

I've seen others have this problem, but havent seen any solution.

I can change tag on a file of 12,8 MB but not on files above 13 MB.

To change the tags I have to copy the files locally, change tag and copy them back, but I hope someone has a better solution to this or knows why this happenes.

The problem seems to be that your NAS doesn't report the available free disc space correctly.

Is there a firmware update available, are there any settings where you can tweak the behavior?