If that doesn't work, try resetting the Workspaces: Reset a workspace Reset the current workspace to return to its original, saved layout of panels. 1. Do one of the following: • Click the Workspace menu icon and select Reset to Saved Layout. • Choose Window > Workspace > Reset to Saved Layout. from here: https://helpx.adobe.com/premiere-pro/using/workspaces.html
If that doesn't work, try Preferences > Audio Hardware and set Input to None.
If that doesn’t work, try updating or rolling back your graphics driver directly from the video card manufacturer’s site.
If that doesn't work, try creating a new project and import the old one into it.
I tried deleting the media cache, but that did not solve the problem. Before seeing this reply, I recieved the suggestion to convert the nests to After Effects Compositions, and that did the trick. I did this by right clicking what was in a nested sequence and selecting 'Replace With After Effects Composition.'
I am running version 14.3.0 of Premiere and Mac OS Catalina version 10.15.7 (19H2) with 2.4 GHz 8-Core Intel Core i9 processing. The source footage is a Youtube livestream downloaded as .mp4, 640x360, H.264, AAC and phone footage as .mov, 1920x1080, H.264, AAC. The problem occurred when I had nested sequences on video tracks 1-3, then added anything to video track 4. I am using Mercury Playback Engine GPU Acceleration.