I find it strange that I cannot join any non-official tournament. When trying to join, I get a message "You are currently playing in 10 tournaments.
If you would like to play in more tournaments, upgrade your account now!".

I calculated that I'm currently playing in 6 user-created tourneys, in which all the games within my group are not finished. Even added official chess.com tourneys with games in my group remaining, the total is less than 10.

When I was a free member, I could participate in any number of official chess.com tourneys + 1 user-created tourney, but when all the games in my group in that user-created tourney were over, I could join another user-created tourney despite the first still had a long way to go..

i've looked at your tournament list. You're playing in 10 official Chess.com tournaments and 15 unofficial tournaments.

You are right - I got it now: I did not notice "show all" link in My tournaments page and calculated from what came up in the first page. My misunderstanding, sorry :)