Message "Not Delivered" to group chat

An interesting problem has cropped up with Messages wherein messages sent to a group chat fail to deliver.

Background: I am part of a large group chat where most but not all participants are on iPhones but a few are not. I can receive messages from the group, but recently, all attempts on my part to send message to the group have suddenly been showing up in green and coming back with the "Not delivered" error, whereas previously they were all being sent with the typical blue background/Apple Message default. This problem arises across all my devices (iPhone, MacBook Pro and iPad).

Messaging to others, and to other groups, works fine.

I have tried:

• Power cycling my devices.
• Resetting my devices.
• Logging out and then back into iCloud
• Toggling cellular data and SMS messaging on and off.
• Updating to iOS 18.1.1

My sister-in-law had the same problem in the same chat group, which she was able to solve by simply power cycling her iPhone; no such luck for me.

Any suggestions?

Thanks!

Carlisle