CRM for Outlook "pop-up blocked" warning when accessing a custom web resource

Hi

I have a client who is using CRM Online with CRM for Outlook 2015 1.1.

There is a button on a custom entity which opens a custom web resource.  If they click on this button using CRM for Outlook, they receive the generic error message "A Microsoft Dynamics CRM window was unable to open, and may have been blocked by a pop-up blocker.....".  However, if they follow the same steps in IE or Chrome, they can open the custom resource without issue.

I have added the address to trusted sites and compatibility list etc.  I have also disabled the IE pop-up blocker and set IE as the default browser.

Is there any thing else I can check?

Could there be an issue with the custom code?

Thanks