Why is the project(-timing) broken? Clips all of a sudden have wrong (double) length

I am not sure what happened. You might be affected by this bug report when changing Video Mode after loading a project:

The metadata DURATION value you shown is not very trustworthy. It is not written by default by ffmpeg/libavformat, which is wrote your file. Rather, some tool must set that value. I am not saying this is your problem or related; I am simply pointing that out.