Premiere won't open old projects and adds only audio of clips on new projects after updating

[Ogden Newspapers of Utah]

Old projects open with nothing in them since updating to Mac Big Sur and the newest version of Adobe Premiere. Also, when starting a new project, the first video clip comes in fine to the timeline then after only the audio of other clips. This works the same no matter what clips I pick. Never had issues before this. Help please....

[Ann Bens]

First issue:

Import old project into new project instead of opening and before doing so

FAQ: Deleting Media Cache files

Second issue:

Most common user error is not source patching the track.
Solved by first opening clip in Source Monitor, set in and out point. Then drag/insert into timeline.