When you upgrade a 1.15 world to 1.16.1, while you're in spectator, the gamemode switcher shortcut F3+N will fail to work unless you enter another gamemode manually.
What happens is that the previous gamemode is not set, rather than having a default value, and F3+N tries to input the value in the gamemode command, resulting in "/gamemode ", and an error in chat.
It's an easy fix, really, make the previous gamemode field default to creative mode if the player enters the world in spectator and doesn't have that field yet.
Thinking about it, this may also affect LAN worlds opened to spectators where the joining player never joined before. Not sure about that though.
Lovely, thanks, this is reproducing MC-190916. Closing that one as duplicate of this because this one because this actually has steps on how to reproduce.