(Sorry to answer my own post again, but the forum always tells me I'm not allowed to edit my messages.)
Actually, it seems like a problem with motherboard placement rather than just the tiny metal tabs from the the I/O shield.
It is just as if my mother board was too far from the back of the case and also a little bit too close of the right panel. (the actual connections are "deep" inside the case and also slightly off to the right than the place they are supposed to be if I see the I/O shield.
This means dust can easily enter the case this way quite easily.
the cover pops in from the inside of the case, don't worry if some of the little tabs came off that's no big deal. As to the alignment look at your motherboard tray there will be many holes in it so that you can fit various types of motherboards you will need to figure out which holes to put the standoffs in in order to make it fit correctly
Thanks for your answer. Unfortunately, there aren't so many holes in the case. Just those to fit ATX and those for mint-ATX (in many cases, the holes are shared).
I'm pretty sure I used the right holes, because otherwise, the actual I/O wouldn't be 0.5cm off, but rather 10 cm.
However, by nudging things a little, I have now come to an alignment that is not perfect, but that I can live with.
Ports are now reasonably easily accessible, and the only problem is that the thing is not a great anti-dust shield, but I suppose I just have to be efficient with my room cleaning ) Come to think of it, it's not much worse then what I had with my latest system, from Packard Bell.
Thanks for trying to help me, and for reassuring me about the little tabs.
My system is now 98% working (only one front USB port not respondiong and some problem witht he sound card, a sound blaster X-fi titanium, recognized by windows but not by creative's own control panel)