But, this "watchdog reset of ERCOS" ....when that happens, the car is like "stoping" for 0.5 second. Can that short time do any harm to the engine ? If it happens on full load ?
No, its like turning off the Ignition for 0.5 seconds and then turning it on again.
There is no critical part in an engines which dies if its not operated electronically anymore all out of a sudden.
This is actually done in a revlimit, fuel is cut of violently thats why the engine rattles quite bad if you bang into it.
And even that wont harm it at all...
The worst thing that could happen is that the KWP2000 load blocks some calculations from being done so fueling/ignition/something else might be inaccurate for a cycle before the reset kicks in. Even in the unlikely case (like 0.00001%) it cases too early ignition (more probable would be too late ignition) and produces knock once this wont harm the engine.
Even more detail:
The functions in the ecus are executed in "tasks" for specific time intervals, for example 1ms, 10ms, 100ms.
These task execute the functions which need to run in that raster. KWP2000 is handled in the 2ms task iirc.
After the 2ms Task has finished calling all functions it needs to call it calculates how long it took for the task to run,
if the KWP2000 function spends too much time in the 2ms task the measured task time for the 2ms task will be too long.
The ECU will then as a safety precaution reset as it can not guarantee 2ms execution of the functions (which depend on that raster) anymore.