Avi,
I was just about to write a message to thank you publicly here.
What Avi found ? switching to Corona Physical Material, with preset Perfect Mirror solved the problem ! thanks a lot for fixing so quickly my problem. However, I'd really like to know what was the problem with the Legacy material here. I mean, mirror material is probably one of the simplest one can set up, so ... did I do something wrong ? or is that sort of a limitation of the Legacy material somewhere ? or a bug ? or ? sorry, by nature, I hate not understanding something :)
Thanks again for your help !!