I've found many conversations about the known error occuring on some 3550 switches during an inventory collection ("device sensed, but inventory collection failed".)
A minimum IOS version must be installed. In my case, I upgraded the 3550s to 12.1(22)EA1.
But on 9 switches, 6 still failed.
I've checked the configuration, the passwords on each one.
Does someone successfully succeed in collecting this category of swtichs ?