Thank you for reporting this. Slow server responses might indeed block the script.
Since the conversion is stepwise, it should be ok to resume it if it fails mid-through. A good thing about Symposium is that you can uninstall it and go back to normal PMs at anytime, and back again to conversations by using the converter. So, if you experience odd behaviors in existing messages (eg.: last messages missing from conversations list), I'd suggest you to run the conversion again from scratch (uninstall, reinstall and convert).