I'm switching accounts, so I did the following:

1. Go into campaign while logged into first account
2. Unassign character
3. Copy campaign's invite link
4. Logged into second account, use invite link
5. Click on the character I unassigned from the first campaign to try and claim it

This worked ONCE.  Now it will not work.  I see my character at the top, I click it, nothing happens.  I see the first character I claimed (for a different campaign) below.  

Any ideas?

I tried to include a screen capture.  didn't work.  you can see it at https://www.screencast.com/t/gHjDodgVNUn if you want.  

..and before you ask, yes I have slots available.  1/unlimited is what it says. 

...whoops!  Never mind!  Didn't see that button at the bottom of this page, sorry, feel free to delete this thread!