After having certainly made a bad manipulation while moving files and folders, I can no longer empty the trash can whose contents appear with a line across.
I quit DTP, checked and optimized the databases, I get no error message.
What should I do to?
Thanks for your advice!
The items in the trash are indexed, as indicated by the little icon to the right. I suppose you not being able to delete them is related to this fact.
As so often, I’m just thinking out loud here: take a look at this post by Jim some while back.
I tried to reset the alerts as mentionned in this post but nothing changed, I’m still unable to empty the trash 
Since upgrading to 3.7, I have what I think may be a similar phenomenon; I think 
After emptying DT’s Trash, when I come to empty my system (11.3)'s Trash, I get the ‘…items in use…’ message. ‘Continue’ doesn’t empty the system Trash.
Strangely, ‘Skip’ does.
Everything I have in DT is imported - nothing at all is ever indexed.
Hold the Option key and choose Help > Report bug to start a support ticket.
Finally I restored a recent backup, sometimes this is the easiest solution.
Thanks to all for your suggestions.
It happens again 
I start a support ticket asap, thanks for your help!
