I can no longer import images from my Canon R5 Mark II into Lightroom (updated) even though it works

Are you talking Lightroom (Lr icon) or Lightroom Classic (LrC icon)? There is no Lightroom 14.2, but there is Lightroom Classic 14.2

I can no longer import images from my Canon R5 Mark II into Lightroom (updated) even though it works with my Canon R3.

Please explain more. Why can't you import? What happens when you try? Please provide details. If there is an error message, please provide the exact unedited word-for-word error message.

Show us a screen capture of your screen when you can't import. Use the "Insert Photos" icon to include your screen capture in your reply.

Try resetting the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com)

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

Does it work in the past?

What has been changed on your system before the error occur?

Do you get an error message? If yes, please post the exact message word-by-word.