From what I’ve been hearing on Discord at least, adding three zeroes to the number of active players online at any given point exposed a lot of software bugs on the server, which the lead programmer’s currently working on fixing, so that may well be a major factor involved.
The paused-and-canceled matchmaking sounds like a failed connection to an opponent, which can be a sign that you need to forward UDP ports 9991 and 19900, but it can also be a sign that your opponent (or the person it was trying to connect you to, but failed) needs to forward those ports. It might also just be related to the server bugs, honestly. Hopefully things should be a lot better in a few days(?) once it’s all ironed out.