Other features or issues related to Microsoft Partner Center
I figured it out. I just opened a support ticket with Microsoft, explaining that I was getting a "The store blocked this account. Ensure the account has permission to read reserved app names and that it is linked to a valid Developer subscription" error message.
I was afraid that MS would ignore my request for help, but within a couple of days, a technician contacted me to let me know that they had cleared the error for me, and closed the ticket. I confirmed that it worked again on my end, and submitted my app shortly afterwards.
Hopefully this approach will work for other developers in the same situation.