Encoder Failure on Render & Replace and Export

Until about 6 months ago, I used to be able to create Premiere Pro Projects compiled from diverse sources, including screenshots, different movie file formats, and audio mixes. Since then, as you can see from the four screenshots below, whenever I try to Render & Replace even a single mp4 project, after the messages 'Analyzing Project' and then 'Transcoding Project', an Encoder Failure error message appears - every single time. In the project folder, a short 'rendered' version of the original .mp4 file appears. The same happens when I try to Export the project to any format. This is occurring on both Mac and Windows platforms across several different machines, including Desktop and Laptop. I have the latest version of Adobe Premiere Pro through my staff account at Edinburgh University.