Problem

A site collection administrator submits a ticket indicating that whenever he tries to share a subsite with an external user, jsmith@acme.com, a popup message appears stating

contoso.sharepoint.com says

Couldn't resolve user i:0#.f|membership|jsmith_acme.com#ext#@contoso.onmicrosoft.com

You check the site collection's User Information List, but didn't find the external username there.  You next check in User Profiles service application, but didn't find the username there either.  Lastly, you check in O365 Active Users and External Users and don't find the external username there.  Next, on your own machine, you attempt to share the sub-site with the external user, and you are successful.  You also note that as you type the username you don't immediately see a match appearing - even after you enter the entire username.  On the other hand, you verify (via Skype session) that on the site collection administrator's machine as he enters the username, a match is immediately presented.

Solution

In this case, Google Chrome was found to be caching the sharing form's: 1) username matching and 2) the response to this particular username.  Completely clearing Chrome's cache - including form and content data - resolved the problem.

References

Notes

  • The site collection administrator had previously tried adding the external user and failed, and I had removed the username from SharePoint Online's User Profiles service application and deleted the user's entry in the O365's External users list.  This should have enabled the site collection administrator to then add the external user to the sub-site.