Equipment calendar doesn't accept invitations

Hello,

We have some equipment mailboxes in our on-premises Exchange 2013 server and a couple of days ago I changed the booking delegate of one of them. Since then, people cannot invite the equipment anymore: I tried to remove the delegate, selecting the automatically accept feature, put myself as a delegate, change the mailbox type to Standard and to Room, updating calendar permission, but still nothing.
Basically, when a user invites the equipment, nothing happens: no reply, no event in the calendar, no notification to the delegate.

Do you have any idea, please?

Thank you,
Luca