wild guess here, but changing the username might've put your main back under the 'new users can't comment for 7 days' status
I guess that could be the case but even prior to changing my username (i changed it to try and fix the issue) it was still a problem.
it's caused by a blocking not working and a snippet of neocities code called "jerk filter" and if 2 or 3 people will block you, you can only comment on your site and your supporter sites from main account.
same i have this problem as well,, randomly started a few months ago!