You did not say what OS it was, but if it is Windows/Vista/Windows 7 you can try the following to find out which program might interfere:
Go to http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx en download process explorer. It is a replacement for the task manager. If you have the problem, start the program and select poser.exe, double click on it. If you do this, there is a tab which says "Threads". If you select that, you can see all the threads and also with which module or program they are communicating. If you see a "foreign" program there, it might be the one which is interfering.
If you close the interfering program, the render will finish. But once you start up this program again, the problem might come back - in this case you have to reboot the machine.

I sometimes have the same problem as you have (twice up to now) and I had to reboot the machine to get rid of the problem