Chatter Free users: Automatic login link works, but username/password login does not

I'm trying to setup some "Chatter Free" license type users in a sandbox. (These are new test users not users copied from production.) The long automatic login URL in the initial (and any "Reset Password") password confirmation emails works and allows a new password to be specified and then routes through to normal access to the org. But when I then logout and try to login via https://test.salesforce.com/ using the username and the new password the generic "Your login attempt has failed..." message is displayed. There is no record of these failed attempts in the "Login History" either. (A typo in the username or password is the obvious explanation but as well as carefully repeating the process I've reviewed this with one colleague and left another colleague to try it independently, all with the same result.) Using the sandbox name as a suffix on the username doesn't help. So I feel like I'm missing something obvious here perhaps related to this being a sandbox or perhaps relating to the license type. Any help appreciated.