Why do I keep getting the error message (425) everytime I try to use the Messages App?

[AC Question]

Until around 3 weeks ago I could use the Android Message App. Now every time I try to send a message using this app I get the following error:

"Failed" "Message not sent. Message saved. Please try again later. (425)"

I can use "Viber" and FB "Messenger" with no problems

Please help...