Unfortunately I'm experiencing even more lockups with 1.5.0 and it's happening with only one probe plugged in. Here's a run down of my experiences from 1.4.0 on.
To test if the BrewBit is locked up or not for a particular probe I have been simply unplugging it and plugging it back in again. If the BrewBit acknowledged the probe being unplugged I consider it fine. Lockups are obviously also reflected on the website since stats are no longer reported.
Yesterday when running 1.4.0, I unplugged one of the probes and the BrewBit reset itself. This matches up with your release notes, except the reset didn't occur until I unplugged the probe. Your notes seemed to imply the BrewBit would reset itself when it detected a lock up. I removed the second probe and let the BrewBit do it's thing over night. It seemed to run fine with a single probe.
This morning I upgraded to 1.5.0 and plugged the second probe back in. Within a couple hours I did the unplug/plug test and both probes were locked up and the heater output had been left running. I don't recall ever seeing both probes freeze before. I reset the BrewBit and removed the second probe. The temperature was obviously a bit high at this point, but I had to leave. So I left the second probe unplugged and assumed it would sort things out. Unfortunately the BrewBit locked up while I was out with only a single probe attached. It also appears to have occurred fairly quickly because the cooling output hadn't even had a chance to kick in yet to compensate for the excessive heating. This evening I reset the BrewBit with a single probe and it began cooling. Again shortly there after it locked up, this time with the cooling output running. I did this a couple more times with the same result. I've unplugged my BrewBit for the evening because I can't risk it running either output indefinitely while I'm asleep.
Also of note is that there appears to be a change in the PID logic in 1.4.0 because prior to upgrading the cooling/heating cycles were frequent and consistent. After upgrading they changed and no longer appeared to follow the same constant cycle as before.
The graph below hopefully illustrates the behavior a bit. The big jumps while running 1.5.0 are the lock ups and the big jump in temperature show where the BrewBit was rebooted and came back online.
I wish I had better news guys.