Scheduled invitations not working

I'm testing the smart lock, and the scheduled invites are not working.
I upload the invite to the phone, tap on the key icon, and the spinner keeps turning endlessly without unlocking the lock; sometimes, after 20/30 seconds, the lock emits a quick double beep (like when the batteries are installed for the first time or when the lock is identified) and the spinner goes on.
However, instant invites work perfectly, as does unlocking via the "Security" app.
The lock's firmware is updated to the latest version, 3.9.0.
The problem also occurs with another phone and with a different lock that I tested and returned because I thought it was defective.