Okay, this is a little off the wall, but it happened to me, so I'll pass it along FWIW. My Linux box got into a situation where the /tmp directory, which has its own partition, contained a file advertising a size larger than the entire partition! (I still don't know how.) But my system monitor reported that /tmp was 80% free, and most programs didn't have any problems accessing it. Netscape's mail client, however, choked every time it needed to create a temporory file, with an "out of space" error. Once I discovered and eliminated the anomalous file, things were back to normal. (A fsck is recommended, BTW.)

If, in fact, your problem turns out to be a file space issue, you may want to give your /tmp directory a thorough perusal -- just in case.