Issue Resolved.

What I ended up doing was deleting the affected character's 'USER' folder in the FFXI directory. Not sure how to pinpoint which is which if you have multiple characters, but I backed up and removed all User folders in the directory, and then logged into the affected character with no issue.

FFXI will create another User folder with the same name as the previous, presumably corrupted, folder. I just moved the other folders back into that directory for my other characters, and replaced the Macro file mcr.dat and everything is back up and running.