gornemant is 100% correct – This is a bug exactly as described after Mojang --> Microsoft account migration and attempting to use "/t" for in-game Minecraft chat.
All Microsoft / "xBox Live" settings are correct for enabled chat. No one has made a client-side or server-side change. I've played Minecraft for seven years, done ITSQA for eight, and been in general IT Ops wrangling things from pseudo-code to server switches for 25 years. This error happens after the account migration.
This isn't a dupe of MC-209432 , btw. None of the steps-to-repro and resolve that bug apply here.