Did you grant SSH access for the subscription?
https://support.plesk.com/hc/en-us/articles/12377931547159-How-to-give-limited-SSH-access-to-Plesk-domain-s-system-user
You can try going to Mail's Preferences, go to Accounts, select the email account, switch to the Server Settings tab and click the Advanced IMAP Settings button. There the IMAP Path Prefix can be set to INBOX. May or may not work, just a thought.
You'll need to get the activation code, which depends on how the license was acquired. Checkout the following;
https://support.plesk.com/hc/en-us/articles/12388069626391
I would probably do it on the network level. A generic deny all rule, and then separate rules to open the certain mail ports that are needed. I'd check with the host and see if this is something the can accommodate.
OWASP is a more restrictive rule set in my opinion.
From the Webserver Configurations Troubleshooter., Set STATUS to Error, Select all, then click Rebuild?
Edit: Also after repair, if any remain broken, remove them.
Might be related to;
https://talk.plesk.com/threads/firewall-disabled.369557/
Have you installed the updated firewall extension from earlier in the month to see if that addresses your issue?
# apt-get update && apt-get upgrade -y
Something tells me you might have an issue with a certain repo, which if so - doing a system update would throw that as an error.