Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.
At some point when we were adding a new site under "Sites" by selecting the "create site in the left column, we were not able to select the folder "sites" anymore. Only "personal" was visible. After some investigation i found the cause.
SharePoint works with so called "managed paths". These paths define which paths are excluded (if you need to access a physical path on disk) and which paths are included (paths managed by Windows SharePoint Services).
You can reach this by going into SharePoint Central Administration, Windows SharePoint Services. Select "Extend or upgrade virtual server" and select the website from "complete list". Under "Virtual Server Management" there is an option called "Define managed paths".
Check if the "sites" and "personal" both are defined under "included paths". Now goto "site settings" of your website. select the option "Manage personal sites" under "User Profile, Audience, and Personal Sites".
Check if "personal" is selected at "Personal Site Location". In my case "sites" was selected which caused "sites" missing. The managed path selected here will NOT appear at the create site page. Now goto the website again and select the the page "Sites". Choose "Create site" under "Actions" and there it is again. Your "sites" folder appears again.
