Mailbox locked

For the past week, Gmail has been unable to connect via POP3 with the following error:

Authentication error.
Server returned error "Mailbox locked."

According to Zoho's documentation, this is due to multiple accounts attempting to access the same mailbox, which is not my problem. Gmail is the only thing accessing this account.

I contacted customer service, who told me to change my password. I changed my password. Still not working. Customer service's response is to ask me to do a screen sharing session, which is not helpful at all. Just unlock my mailbox. Prevent this from happening again. If I cannot access email via POP3, this service is worthless.