I was having the same issue. I worked with technical support for about 3 hours a couple days ago and never totally figured it out. They took control of my computer and remotely tried out several things. In the end we downloaded Premiere Pro and exported the project from there, just so my current project was completed. They said a new update to Rush is coming to address this issue. I hope that wasn't just a line to make me feel better 🙂
There was perhaps a video compatibility issue happening for me. My video was taken using my iPhone. I tried putting the iPhone in High-efficiency mode and in Compatibility mode. I ran into unique problems with both. For now I am supposed to used Handbrake to transcode my iPhone videos before importing them into a project. Unfortunately that isn't an option if you are already deep into your edits. I've also edited several video without issue using the exact same method and equipment. It's very frustrating. hwat equipment and format are you using?