Error 916 strikes me once more, but it happens now again after a /npc command. I've tried redoing the client, redoing psobb.exe, and all that other nice stuff. I need a bit of assistance o;

You made sure that your characters are using the right default character settings (head/body/etc), which is necessary for the /npc stuff to work properly?

Note: A quick forum search turns up this, this, this, and this in bugs/help, all of which addressed this problem in the past.

I cannot even access the Character Select screen at the moment.

/npc issues generally are resolved by that, so unless you're missing the necessary skins/data (in which case, PSOBB would crash when trying to load or display your character) or having connectivity issues, I really don't know what to say.

Do you have hamachi or some similar service installed? - if so, can you uninstall it?

Hamachi was recently uninstalled. Nothing else should be causing it, But I'm (still) having a bit of issues with the connection.

My immediate suggestion would be to try doing a clean installation of PSO (if you have any custom skins/music installed, be sure to make a note of that before deleting the folder!).

Also, double-check to make sure any programs you have running in the background aren't interfering with PSO's connection (Firewalls, etc).

For some time now, I've been contemplating putting together a clean installation for anyone that needs one and doesn't feel like going through the process of having to compile everything, all over again. If there's already something like this, I'll avoid doing it entirely. Otherwise, with the permission of the admins, I'd like to do this and host it on my web server.

You can try editing your mtu to 1420.
This'll do it for you if you like.
http://www.mediafire.com/?oq2rvz1v60vvk2c

It so happens that I was able to get in, after uninstalling an antivirus (weird, right?)
So I'll be on a bit more frequent now.