New message window idles after Send action.

[lorenzoabel]

Hello everyone!
This is my very first post here, so be gentle!   
I have Roundcube 1.4.2, running on Ubuntu 18.04.3.
Our directory is LDAP (Windows Server 2012)

When a new message is directed to a group with a large membership, let's say all staff in the organization, after the SEND button is hit, the new message window idles (spinning wheel of death) and never show the "Message Sent" notification, nor closes the Window.
Curious enough, the message is though sent to everyone!
Ideas?
Thank you!