Plesk and sites are not accessible: 503 The Module DLL C:\Windows\System32\inetsrv\pleskurlprotection.dll failed to load. The data is the error

C:\windows.old directory exist on the server

Incorrect permissions on Microsoft C Runtime Library files that can be found with Process Monitor:

The permissions could be changing constantly. In this case, ApplySecurity.exe is involved in the issue.

Temporary W/A:

1. Connect to the server via RDP

2. Change the deny rules to allow on all C:\Windows\System32\api-ms-win-crt* DLLs for psaadm, psacln and psaadm_users:

3. Change the deny rules to allow on ‪C:\Windows\System32\ucrtbase.dll for psaadm, psacln and psaadm_users.

Fix permissions by comparing them with the test server on the file affected. If it is not clear who is changing the permissions and if this happens constantly set up a file audit. See the example of such an issue in https://jira.plesk.ru/browse/PPS-12359

As a troubleshooting step, check if there are some Windows.old folders - Windows Server may treat such folders are a part of the existing operating system golder and running AppySecurity.exe on them may result in changing the actual permissions.