Unable to add new subscriptions after installing SmarterMail: mailmng failed: Incorrect Content-Type: must be textual to extract_string, JSON to extract_json

Symptoms

• After installing SmarterMail, Plesk is unable to add new Subscriptions
• The following error message can be found in the Plesk log at %plesk_dir%admin\logs\php_error.log:

CONFIG_TEXT: [2023-01-30 07:56:18] 18884:63d7e894d8cae ERR [panel] Long task executor: id=91 completed with error: mailmng failed: Incorrect Content-Type: must be textual to extract_string, JSON to extract_json.
at newDomain(example.com)(SmarterMail::Local::Provider::newDomain line 195)
at Unable to execute console command: '--add-domain'(vconsoleapp::start line 95)
at Unable to execute command: '"C:\Program Files (x86)\Plesk\admin\bin\mailmng" --add-domain "--domain-name=example.com" "--disk-quota=104857600" "--total-disk-quota=-1"'(vconsoleapp::run line 117)
(Error code 1):

• The same error may appear during an attempt to remove a mail account 
• The default http://localhost:9998/ binding exists for the SmarterMail site in IIS (Windows > IIS > Sites > SmarterMail > Bindings)

Cause

ModSecurity blocks the newly created default SmarterMail website and prevents the addition of new Subscriptions In Plesk as a side effect.

Resolution

1. Connect to the server via RDP
2. Start a command prompt as Administrator
3. Run the following command in order to reconfigure ModSecurity:

C:\> plesk sbin defpackagemng --update --type=waf.modsecurity --package=modsecurity --fix

Once ModSecurity is reconfigured, it will no longer block the SmarterMail website and the issue will be resolved.