Well - the good news is that I was able to import them all first time...
So - the most likely cause is going to be timeout on your server. With such a small forum I am surprised but if the timeout on the server is set very low then that could be it. To be fair, the import script is not that well optimised - but as I say - will usually import a far bigger forum than yours.
If you could try making the timeout value bigger I think you should be able to do it.