[FIXED BUG] Cannot open dedicated IIS Application Pool page Error: Unable to manage the IIS application pool because all websites are using a single dedicated pool

Symptoms

Cannot access and manage IIS application pool settings at Domains > example.com > Dedicated IIS Application Pool for Website.

PLESK_ERROR: Error: Unable to manage the IIS application pool because all websites are using a single dedicated application pool
The server-wide policy is set to Always assign one application pool to each subscription at Tools & Settings > IIS Application pool > Global Settings.

Product issue:

#PPP-45368 "Clicking “Dedicated IIS Application Pool for Website” no longer results in the “Unable to manage the IIS application pool because all websites are using a single dedicated application pool.” error on Plesk servers with the “Tools & Settings” > “IIS Application Pool” > “Global Settings” > “Assignment and placement policy” option set to “Always assign one application pool to each subscription”."
Fixed in:
- Plesk Obsidian 21 January 2020 (Windows)

Please consider updating your server:

If update is not possible for some reason you may try the following

workaround

As a workaround use the following server-wide IIS Application pool settings in Plesk:

Log in to Plesk.
Go to Tools & Settings > IIS Application Pool > Global Settings:.
Set Place websites in a dedicated application pool by default and allow shared pools and Apply the changes.