This may be a bug in Mantis, or it could be in the iPhone OS. I have this problem with a bunch of different apps (or sometimes all of them) and then when I reboot my phone everything works again. Ugh.
As for Mantis, if you have a spotty net connection, this could be a problem. Just go into the Settings page, and turn of "Check for Updates." That'll solve your problem, but you'll need to manually "Get Updates" every time you purchase a new Bible for Mantis.