I tried “offlining” the suspect media, but it didn’t solve the problem.
So I made a new project and duplicated my most recent sequences into it. I then deleted a suspect sequence, and the problem went away!
I still have media on the drive from this corrupt sequence, and this media is in almost all of my duplicated sequences.
So it seems that a corrupt sequence was screwing up the project. but i’m not sure…
My new project has a lot less items in it. So maybe this helped?