I had this issue the first 2 times. I signed in as guest, played for a little bit, got an error about memory allocating, got kicked off game. I played as guest 3 more times then tried signing in with an email. The email already had an account, but I didn't know password, so I registered with a new email. Got in fine, username and password are saved but I keep getting kicked off with this video memory error.

Clicking register went to just a blank screen that you could hit esc on and go back to the previous menu.

I also had a similar issue where the register button was not responding. I thought it may be AdGuard Home(so I disabled it temporarily) blocking access to the servers. I also tried another password without the plus symbol (+), but then when I tried to register again, it said username already taken, so I re-enabled AdGuard and I tried to login with my original password and got logged in. Not sure if it's combination of AdGuard, password characters, or server/other issues.

When I tried registering, I was able to fill out the requested fields but then after clicking the Register button, nothing happened. I exited back to the main menu and tried entering the credentials that I tried creating and the game didn't recognize them. At that point I tried clicking Play as Guest and the game said I couldn't because I wasn't a "unit leader." I exited the game completed, re-opened it, and just hit Play as Guest and that worked to get me in. Still not registered, though.