In some instances widevine will cause OBS studio to capture a blank screen.
There's not really a good way to work around that in software that is publicly available.
However, if you are willing to put the time and effort and you can always play the video into an HDMI splitter and then have one of the outs going into another computer that has an HDMI to USB adapter.
If the HDMI splitter is cheap enough it may break copy protection on its own and output an unfiltered signal that the USB adapter can then ingest. This takes a lot of time and creates very large files that then have to be re-encoded via handbrake or similar app, but if you have a single video or a couple of videos about your worried about losing forever and you absolutely have to have a backup that is a way to do it.