Whoops! Looks like a bug I fixed a few weeks back in our fancy new feature branch has bitten a lot of people over the new year. The fix is now in the b1.3x branch, along with some other nice improvements (like having CardDAV sync now actually work in an ongoing way).
Sorry about that folks! I hope it hasn't impacted too much on your new year.
Get your update from the Android Market - or however it is that you get hold of your copy of aCal.
In the process I notice that there are now just over 1000 people who have purchased aCal through the market! I guess that means we'll have to be extra careful to make sure we don't have any regressions when we switch releases over to the new branch in a few weeks time.