Shower goes cold, hot water supply gets cut off completely?

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Brussels, Belgium
I have a new shower that has worked fine, albeit with low hot water pressure, for a couple of months. Now it starts hot, but then goes cold completely, sometimes after a couple of minutes, sometimes after less than 30 seconds.

Meanwhile, on the side of the bathroom sink, there are no issues with either hot water pressure or hot water delivery.

So, I tried closing the hot water supply on the boiler side first:

1) Sink: No hot water at all (as expected).
2) Shower: There was still some cold water coming out of the showerhead with the thermostatic valve set completely to hot.

I then went back to the boiler and reopened the hot water supply and closed the cold water one.

1) Sink: Again, there was a constant supply of hot water with good pressure.
2) Shower: The water started coming out hot (low pressure, though), but then, after 15-20 seconds just stopped, as if I had shut off the valve completely.

What is going on here? Any advice would be greatly appreciated.

P.S. The combi boiler is a Viessman Vitopend 100.
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