-Both my children have this issue.
-Deleting save data and using new Microsoft accounts hasn't fixed anything.
-Using a new Nintendo account HAS fixed the issue but this would mean buying the game again and getting another online subscription. I was able to test this using my own Nintendo username on my sons switch but this is no kind of solution since we can't play together this way.
-The issue seems to be connected to your Nintendo account, can others confirm this?
@Christina Lee Thankyou, I have let my son use my Nintendo account since I have a copy of Minecraft on it for him to use. I can't do the same for my daughter because It would require a new nintendo acccount and a new copy of minecraft which im not prepared to pay for, so unfortunately she still has to load in for a long time and sometimes reboot once she does manage to get in.