It happened with any server, regardless if it had a custom map or not, and another friend of mine had the same issue as well. We tried the waiting game to see if it’ll eventually load, but like I mentioned, the game just crashes and closes out.
But, what I found out is, both of us have a fairly large amount of mods installed into our games. I had the random thought of removing them all before I started MP. Turns out that was the trick. Did some testing with the mods left in, and yeah, seems like MP is trying to load into a server, but removing all that has solved my issue.