377a Water Pump Blocked: that's how it started with mine, as well. Replacement needed urgently. Normally you should also get an engine light and with higher temps also a massive loss of power with max 60kph. Not a cheap fix, if you have to pay the dealer. The (electric!) pump is also not cheap.
When changing check the thermostat and reservoir (the black bucket on the left side of the engine bay). The reservoir is known to break/ leak, especially with higher pressures after the pump blocked.
I would change both nonetheless.
I know no other N54/N55 owner to not have replaced the WP. Normally they last until somewhere around 100kkm. Mine lasted until 95kkm and went bust in the Italien Dolomites (ruined the weekend, as the failure popped up on Friday late afternoon )How prone to failure are these pumps? do they all eventually fail and need replacement or only some?