Choose "Console" in the "Debug Window" found under the "Help" menu.
Enter the following command:
This will get from your wallet the transaction that it is attempting to push to the network. It will be a long sequence of numbers and letters between A and F.
Paste that entire sequence of numbers and letters into a post here, and we can look at the transaction and let you know what is wrong with it.
There is nothing in the results of getrawtransaction that would allow anyone to take your bitcoins, but it will reveal the bitcoin addresses that the transaction is sending to as well as some of the bitcoin addresses that you've received bitcoins at in the past. If you do not want these addresses to be public knowledge, you can send the transaction via PM or email (and can encrypt it first if you like).