I am having the same issue with my S9. It will work on the first group message, then give me the "invalid address entered" error after that. I have tried in safe mode and get the same error. If I restart, the first text will work, but once someone responds, I get the error again on my next reply! I've had this phone for two years and have not had an issue until this started.
@userED8SpqbECi Please try wiping the cache partition.
My phone started giving me the error mentioned above again. Is this partition clearing method the only known fix to this issue? It has been 2 hours of working without error and then it's like a switch is flipped. That is a lot of steps to do multiple times a day.