We recently came across a problem in Microsoft Dynamics CRM 2015 where the Converting Activities function was not working. A customer noted that on the Appointment, Email, Letter, and Fax Activity forms, the "convert to" menu link for Opportunity and Case did nothing. On other types, these menu items worked as expected. We worked through checking the following browser settings - popup blocker, compatibility view, and trusted sites - all were correct.
Since this was discovered in a CRM online system, we contacted Microsoft support for help.
After some review, the solution was to enable legacy form view from the Settings area. This issue is a known CRM "bug" that will be fixed in either CRM 2015 UR 1.2 or in CRM 2016. Until the roll-up or upgrade is available, the legacy for view is needed. There is no code on the activity forms, but there may be other features that were brought forward from previous versions of CRM forms, causing the problem.
Beringer Associates, a Microsoft Gold Certified Partner, is always here to provide expert knowledge in topics like these. Please contact us with any questions you may have.