This is my first post on this forum. I hope it will help you out.
I don't think your problem is with the tankless heater. You may be getting a cross-over of cold water into the hot water piping system. Check to make sure there are no faucets open in the house. You should also turn off your valves to your wash machine. Older wash machine solenoid valves can sometimes allow water to pass back and forth between the hot and cold sides due to weakened solenoids. A cross-over can make you think that your running out of hot water when you just have cold water mixing with the hot water.

The required minimum flow rate is 0.5 GPM (0.7 GPM on NC380 model)

I think this is a low-flow problem. This time of year, with hot weather, the cold water may be rather warm. In the shower, you use less hot water right now.

When you first turn on the shower, more hot is demanded initially to heat up the boiler and pipes. After a few minutes, the pipes and boiler are hot enough that you or your thermostatic shower valve mixes in more cold, and eventually the flow rate drops below the minimum. Turn the shower all the way hot and see if the heater turns back on.

If that works, next try turning down the temp on the heater to about 105-110. Just do this during periods of really hot weather; turn it back up again to 120+ to prevent Legionella bacteria from growing in the pipes.

Another thing too, I always take out the flow restrictor in the shower head, drill a 1/8" hole through the center, then replace it in the shower head. This ensures I get a nice shower and keeps the tankless running at least 1/2 GPM. It does not waste water. I measure the GPM in a 5 gallon bucket, it is still below the EPA mandated 1.8 GPM limit. My incomming pressure is 60 psi, all copper pipes, 3/4" main lines for hot and cold. The flow restrictors are total overkill. Seinfield nailed it in that episode where they all went and bought shower heads off the street.

Finally, make sure the shower valve cartrige is clean. There may be a screen inside that gets clogged up with debris over time. The shower pressure drops gradually, and you may have noticed the shower doesn't feel the way it used to.

This is the simplest possible cause of the problem you may be experiencing. Try these first, Good luck!