Here is what it was for me and it goes with what host stated.
This is the key thing to look for that made me look into it.
Are you still seeing a guild chat for a guild not in your list of guilds?
If so that is the one you have to rejoin and then ask them to kick you. It will then let you make any guild on your list your primary.
It was with one of my alts I had the issue but could be with any Toon. Alt guild assignment (Solved)