Problem with adding additional user

I am usually quite tech savvy, but for the life of me do not succeed in adding a second user to my Index 2. Issued the invitation email and my partner’s phone recognizes the scale but then hangs in the next step (connecting?). Tried it all: removed bluetooth and back, removed batteries, reset the scale. Tried to install my partner first, and then it will get hung up when adding my phone as second user. Changed WiFi network. Waited. Must have been some 20+ attempts. 

Need a hint (and if it to return the scale). Thanks!