When I earlier reported that 0.13.0 fixed this, I was unfortunately wrong. It seems that I was just accidentally lucky. Like 1 in 20 attempts works. The 19 out of 20 attempts still do not work. Can't see any pattern in the working attempts.
0.13.0 seems to have fixed the multiplayer problem.
A-ha. Figured it out: if the new sneak button is on, flight is not working.
When I earlier reported that 0.13.0 fixed this, I was unfortunately wrong. It seems that I was just accidentally lucky. Like 1 in 20 attempts works. The 19 out of 20 attempts still do not work. Can't see any pattern in the working attempts.