Intermittent hot water - Bosch PowerStar AE115 Tankless Water Heater...

Sounds like you need a new Thermo coupler.
Could be as simple as a dirty pilot and needs cleaning but I suspect a thermo coupler.

check that the diverter valve operates

The water inlet to the heater must be plugged.

Call a qualified service tech immediately.
Gas explosion indicates dirty combustion parts, and gas is not fully igniting when tankless turns on the gas. So gas builds up and then explodes causing serious damage to tankless, and serious risk to homeowner.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

check your element's,possible one may be burnt up.also your thermostat's should be set at same temp.

will still be under warrent call supplier

check the screen on the cold water intake on the cold side,, you might have to flush the system out, and do you have the shower head water restricter in, if not you may be using to much hot water,,, how many other hot water fixtures are running at the same time? try running just the shower,,,if this is a tank water heater the dip tube could be bad that is on the cold side it takes the cold water to the bottom of the tank and forces the hot water out,, if you ar only getting a couple of min of hot water and the dip tube is bad it is not going to the bottom but just at the top and not getting all the hot water you will need a plumber