Authentication prompt when using SSL certificate

Hi, 

I hope this is posted in the right place, have moved it from another place as I didn't get any responses.

Running SFOS 17.5.0 GA and we are currently seeing the following pop up when users log in that have the SSL certificate installed. Wasn't occuring before on 17.1 but is now. Users with exactly the same permissions  and without the SSL certificate are not seeing this.

Can replicate simply by logging off and back on, waiting for the CAA to launch and then clicking Start. Other boxes appear elsewhere as well but this is the most frequent. 

Anyone else experiencing this at all?