"no ack" errors are pretty much always because of some signal noise between the device and the PowerLinc. You might try moving around your access point to see if you can get a better/clearer signal. Check out the
signal troubleshooting page for more things to try.
It's highly unlikely that it has anything to do Indigo Schedules, though if that schedule is executing around the same time that other Insteon things are happening it could be that you've got some signal collisions that are causing problems.