After having cleaned my mac with some antistatic foam product, weird keyboard problems started happening:
with keyboard viewer, pressing CTRL does not enable CTRL, however if I press LEFT SHIFT, it enables BOTH SHIFT KEYS AND CTRL. This quite a handicap because I use modifier keys regularly.
(I did reset the SMC and the PRAM, noticing that the combo SHIFT-CTRL-ALT-POWERBUTTON (PRAM combo) works, which makes me think the issue is at the software level, not hardware.)
thanks for your answers :)