I don't know what's wrong exactly (havn't checked the code) but it looks like when you have the same wallet open twice or more and put in POS mode on at least 2 machines there could be a "feature" that makes your transactions sometimes go awry.

I ran into the same problem and fixed it by reconstructing my wallet (removing all former transactions, either you have an old backup from before these transactions or you recover the private key and reconstruct a wallet from there [there should be some howto describing the procedure on the net]). Everything went back to normal, at least in my case.

Therefore: if you have the same wallet open twice or more and in POS mode, then close all other instances before passing a transaction.