The live and playback videos are working. Clearing the Java and browser caches solved the final issue.
(I am running Windows 7 and use Firefox. IE11 was getting a similar error, but I actually never use it.)
Thank you for your help. I have to say, this is the best (fastest) help I have received on this forum. I am pretty sure you have been here for a number of years. There have been times (on other parts of this forum) when I felt like it was dead silence after posting a question or problem. I have many D-Link products (cameras, switches, routers, network recorder) and like being able to keep to one brand to reduce the possibility of conflicts between hardware and software. You have restored my faith regarding the use of D-Link products.