I think after tonight's bath the vent looks a bit better and I manage to get some crumbling stuff out of her vent.
It looks like it was calcified so impacted may be right. at least, she is not blocked and seems happy enough scratching around with the other two.
I found her exiting one of the nesting boxes a couple of times with no result. So maybe she was trying to move something.
I'll come back with regular reports.
Thank you for all your help.