My users were going to their My Content area for the first time (ie: creating their personal site) and got the following error:
There has been an error
creating the personal site. Contact your site administrator for more
Checking the ULS, I see the following:
My Site creation failure for user '[some-user]' for site url '[some-site]'. The exception was: Microsoft.Office.Server.UserProfiles.PersonalSiteCreateException: A failure was encountered while attempting to create the site. ---> System.ArgumentException: Another site already exists at [some-site]. Delete this site before attempting to create a new site with the same URL, choose a new URL, or create a new inclusion at the path you originally specified.
It turned out that my Personal Site Location had been changed from "my/personal" to just "my" so it was not pointing at a wildcard inclusion managed path.