I had this problem, too. I'm still not sure what caused it but I know how to fix it.
Java/LW thinks the given save directoy is write protected, which is not really true (you can copy files in it on Windows at least, though when looking at the Properties it shows it as being write protected, which doesn't mean too much because I still use FAT32 which doesn't have real permissions and all folders show up as write protected - at least for me).
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Anyway... make a new dir, move the contents of your old save directory over to the new dir, delete the old dir and rename the new dir to the name of the old one.
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Takes like 10 secs and should work.

At first I thought my Linux changed the permissions on my fat drive but now that many ppl get this problem I guess somewhere along the 2.8.x line LW fussed around with the permissions of the save directory.