Private Messaging plugin - cannot scroll the Inbox after selecting a message

Please regard this as urgent!

I have your Messaging plugin installed. My customers are unable to use it, however, and are asking for their money back as they cannot scroll the list of messages received after they have selected a message. This is an essential part of my elaerning site.

I have the latest version of the plugin installed. I've seen a message on your forum about this before so it seems others have experienced the same problem. See https://premium.wpmudev.org/forums/topic/no-scrollbar-in-private-messaging-inbox-formatting-question#post-1202607

I have granted you Admin access and would be grateful if you would log in to my site to see if you can identify a solution.

Many thanks!

PS I have tried (on another of my sites) deactivating all plugins except for Private Messaging and using a default theme (2017) and the problem remains in both Chrome and IE.