Previews are gray squares

Apple changed the security requirements in MacOS Monterey. Go to MacOS 'System Settings - Security & Privacy' and add Lightroom Classic and Photoshop to "Full Disk Access". Click on the lock icon to unlock the settings, then click on the plus icon to add Lightroom Classic and/or Photoshop if it isn't already listed. If it is listed, then just enable the checkbox. See also Quick Tips: How to give Full Disk Access to Lightroom Classic on macOS

Your previews are probably corrupt. You can delete and rebuild the previews without harming your catalog.

To delete the previews - locate the folder containing your catalog, then select and delete the package ending '.lrdata'

To rebuild the previews - choose Library > Previews > Build Standard-sized Previews

The first thing that you should try is to switch off the GPU support from the Lightroom preferences and check if that helps to fix the issue.

Go to Lightroom > Preferences > Performance tab > Uncheck "Use Graphics Processor" > Restart Lightroom.

https://helpx.adobe.com/lightroom-classic/kb/lightroom-gpu-faq.html#troubleshooting

https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html

If this helps please check if there's an update for the graphical device driver is available. 

More informations you'll find here: Gelöst: in de ontwikkelmodule zijn alle foto's wit – Seite 9 - Adobe Support Community - 13006731

If this doesn't help, you should check your montor profile. It may be faulty.

How do I change my monitor profile to check whether it’s corrupted? | The Lightroom Queen

Another point could be missing access rights.

Please see here: 🔎 Quick Tips: How to give Full Disk Access to Li... - Adobe Support Community - 12913413