After upgrading to 5.8.2 or 5.9.x the following error may be displayed while accessing the PMAdmin site:
“HTTP Error 404 - File or directory not found.
PMAdmin’s 'Forms Authentication' information was not updated correctly during the upgrade.
1) Launch IIS and navigate to the PMAdmin site;
2) Within PMAdmin,, open the Authentication prompt;
3) Make sure ‘Anonymous Authentication’ and ‘Forms Authentication’ are enabled;
4) Right-click on ‘Forms Authentication’ and select ‘Edit’;
5) Confirm and adjust the configuration to resemble this one: