I see the event and can choose an end time, but whenever I do, it resets back to the main PVE menu and my slot hasn’t been selected.
That’s because the first time slice hasn’t started yet. Try again in 10 hours. 
Is your name Will Smith?
That happens to me a lot on Steam. If I exit and restart, my choice is there. So, yes, it’s a case of “have you tried turning it off and on again”.
Happens all the time on android also.
As Moon Roach says exiting the game usually does the trick. When you open up the game again the first node is usually there and playable.
This happens almost every PVE. You are Steam player for sure. The problem is, game registers you even for first time. You just need to restart Steam.
I encounter this problem a lot myself as a Steam player. You only need to exit MPQ program and relog back in. You don’t need to exit steam itself to get the refresh going between your account and MPQ server.
It’s fixed now but yeah I had tried that. I imagine it had something to do with the server issues.
Nope, iOS.
Happens to me on ios sometimes too, I just keep selecting it until it lets me in. Sometimes takes 3-4 tries.
i can confirm this happens to me too. just be persistent and it usually works eventually.