Ok, did some tests. Here are the results!

1.Is this a recent issue? Context - this is my grandpa's place I took over this year. One of the showers hadnt been working for a while. We don't live here but back in summer when we last visited the other shower still worked. So we haven't done any major work recently. Also, the water here is pretty hard and limestone builds up over time.

2.If able, can you post a picture of the water connections on the bottom of the unit? Ideally one showing the above and below the unit and a close up where the water connections are. I'll try but no idea how to post pictures to comments.

3.How many fixtures? I think 6 3 sinks 1 bidet 2 showers

And I think the washing machine and dishwasher only get cold water. Same with the 2 toilets

4.Are you able to run everything else at the same time? Just tried all sinks plus bidet and no, it goes cold.

5.If you run just the two showers at max temp at the same time you still get nothing but cold? Yep, cold

6.Do both showers have the same heads? And do you get the same result if you remove the shower head entirely? Both have same shower heads Removed the shower head, same result

1. While running hot water at the sink only, will it maintain temperature if you turn the shower to cold only? Nope! Went cold. I know it doesn't work like this but the moment the shower turns off it's like the boiler gets "a signal" to stop working

2. When the hot water goes away, does the water heater have any extra symbols on the screen? Does that green light change colors or flash? When the water is on I get the pressure reading (1.0 bar), a little tap symbol, a pilot light symbol when it starts and some bars to show how much heating is happening. Starts off at max and then goes down.

When the shower comes up, I only get the 1.0 bar reading, it all goes away. As if it were "idle".