I could finally sort it out by deleting the configuration and clicking on the debug button arrow and selecting Debug As > C/C++. But it wouldn't work just by starting a new debug configuration.
That doesn't work here, mate.
Not even re-installing LPCXpresso.
Neither moving project files (cleaned) to a fresh folder and changing workspace.
TG it's free... poor chaps who paid a penny...
Removing breakpoints didn't help as well, as pointed in another thread.
Well... finally... click on the Debug down arrow, then Debug As > 1 C/C++ (NXP...)
Or else in a whole year nobody else has tripped on this or users don't come here or there aren't so many users.
If all else fails RTFM.
Debugging is fully described in the Manual. It is different to standard Eclipse.