Some ORF files unsupported or damaged, they used to be OK

It sounds like the issue might be with how Lightroom Classic is handling your ORF files rather than them being truly damaged. Since Windows Photos can display them, it's likely just showing the embedded JPEG preview rather than actually processing the RAW data. That could explain why Lightroom is struggling with them.

A few things you could try:

1. Check OneDrive Sync – If your ORF files are stored on OneDrive, make sure they are fully downloaded to your local drive before opening them in Lightroom. Sometimes, cloud storage keeps them as placeholders until accessed.
2. Try Another RAW Viewer – Olympus Workspace (Olympus' own software) or another RAW editor might be able to open the files properly, confirming if it's an issue with Lightroom or the files themselves.
3. Test with a Fresh Import – If these are older imports, try copying a few ORF files directly from your camera’s SD card to your desktop and importing them again.
4. Check Lightroom Preferences – Make sure your preferences aren't set to treat JPEGs and RAWs as one stacked file. If they are, try changing that and see if Lightroom can separate them.

If the files turn out to be corrupted, but they still open elsewhere, tools like Stellar Repair for Photo might help fix minor corruption issues. But first, I’d recommend checking if the issue is just with Lightroom’s handling of the files rather than actual damage.

 I had been quite happy shooting with Raw+Jpeg mode, but now I wonder. Lightroom does not allow me to unstack the ORF and JPEG, so I can't even view them in Lightroom, they appear as a blurry images. 

I'm addressing this separately since it's a different question.

Not sure what you are trying to acheive. It sounds like you have your preferences set to not treat JPEG's found with RAW files as separate files. When I'm traveling, I'll shoot ORF and JPG in case I want to quickly send a copy out on social media. When I import into LrC, I only import ORF.

You can always export a JPG from ORG in LrC. The difference between it and the one created by your E-PL5 is that it will not have any of the camera settings that may have been applied in camera to the JPEG.

Don't know what you mean by they appear as blurry images.

Lightroom Classic 10.2 Camera Raw 13.2 Win 10 

I am able to view them with Windows Photos both the Raw and Jpeg file and they look OK there, so apparently they are not damaged. 

A RAW file is not an image but merely the recording from the sensor.  It has to be converted into an image which is where LrC may be having problems.

I don't know when you use Windows Photos if it displays the RAW file, unless you have installed the RAW file extension. It may be only displaying the embedded JPG for the ORF. As noted, there needs to be a conversion step.

As dj_paige suggests, it does sound like file corruption. I'd suggest trying to view the RAW files with Olympus Workspace or another viewer that you know will do a coversion from RAW.

If the message says "unsupported or damaged", why do you conclude that the files are unsupported? They are most likely damaged somehow, almost always due to a hardware malfunction somewhere.

Try restoring a backup copy of the photos.