I found a solution and I know why your old videos aren't working.
It is the fault of ipfs since it will only load newer videos. The best way to fix it is to watch your video from ipfs it will take a long time to load but afterwards the video should work.
I'm totally new here - how does that work, watching your video from 'ipfs' ... ? My daughter has uploaded a couple of her animations videos and she's getting that same error. :/
From your Steemit blog post that is associated with the relevant video, near the bottom of the post there should be > DTube on one line and > IPFS on the next. Click the IPFS one and go fix yourself a sandwich or something. It will take a while.