Plug-Ins Not Working and Nor Is White Balance Dropper in Develop Module

Secondly, in the Develop panel (Basic), the WB eyedropper no longer works. If I select it and run it over the image, the "tiles" don't change as I move it over different parts. The tiles remain as per the point at which I first touched the eyedropper to the image. Again, any suggestions for resolution of this, please?

I highly advise that you go and create a brand new post in Discussions on that. When you do, include:

• A nice clear subject like "LrC WB Selection Tool Sample Area Info does not change"
• A complete description of what is occurring . Use the words "White Balance Selection Tool" and "Sample area in Grid does not change while moving cursor"
• That screenshot showing that you do have that option on
• Perhaps a video capture of the issue
• Your System Info as LrC presents it (members will ask, and providing such info is in the community how to and tips)

And in your current post, after you create the new post, reply that you have created that new post (and provide link) so that anyone stumbling over this post will know to look their instead.

View this document for a discussion on Adobe LrC file locations. At point is the presets. And at point is if stored with catalog (the difference)

You may find that those missing presets for the external editors are in the default file location, and if you want to continue with storing presets with catalog, that you will need to copy them from one folder to another.

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

For MACOS, the location (normal) for the modules and for the external editor presets.

Does  anyone have a link showing where to find them?

I think the Author will need that bit of info.

The plugins you see under 'Edit in' are not the plugins themselves, but presets to run the plugin with certain settings. I wonder if there was something else you did, but did not tell us, like change the 'Store presets with this catalog' setting. That would explain it. Writing to XMP does not have anything to do with this.

 I was advised to tick the "Automatically write changes into XMP" box in Catalog Settings. It then took around 48 hours to sync my 35,000+ images. That was no problem. I then restarted LrC (as advised) and found that almost all of my plug-ins have disappeared....

I doubt if that has anything to do with your problems. But nice to see info about what actions were taken. Good practice.

My first reaction to something going this wrong, the messed up plugins would be to consider resitting the preferences file. But I am not all that sure that the preference file is used by LrC to know what plugins are installed as opposed to simply reading the modules folders contents and the Export Editor Presets folder (in AppData on Windows) upon startup. (perhaps one of those folders got fouled up??)

More on that below.

Have you tried simply reinstalling the DxO NIK collection? (P.S. if you attempt this, take this as an opportunity to make sure NIK Collection, etc are up to date). Most post processing software installs the plugins for PS and lrC UPON INSTALL, a few like the Skylum products have a menu entry to do so. Or you could visit the third party's website to find out how to manually install the plugins. But a reinstall the External Editor like NIK is simplest.

Secondly, in the Develop panel (Basic), the WB eyedropper no longer works. I

Might return to this reply after looking

Yes, when you have the WB Eye Dropper selected, down in the bottom toolbar, notice the Show Loupe check box. Needs to be checked.

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A separate commentary about the preferences file and resetting it.

The preferences file contains a information on how LrC should load, on changes to default settings (especially those in preferences) and if corrupt, it can cause all sorts of issues. Now the preferences file does not contain each and every possible setting, but enough to cause very odd behavior if corrupt. You may have noticed during LrC startup that the program reads the preferences file (and sometimes fails at that point) I have no knowledge of how it gets corrupted. Mine never has (knock on wood). But when it does, it does not get replaced via updates, via Upgrades, or even the normal (simple) uninstall/reinstall (not sure about the Creative Cloud Cleanup tool, that's more complete on uninstalls).

Advising a member to reset it may be a lot of a magic bullet a panacea, and might be advised to often too quickly. I for one have advised accomplishing that only to find out that simply deleting a preview file (like one for history) would fix an issue. Or that (on a Windows PC) a simple reset (not a shutdown) would clear out garbage in the Windows Kernel. So I am hesitant above in advising that. (and if I remember correctly, I once had a plugin issue with a external editor that was fixed by reinstall of the third party editor)

If you choose to attempt resetting the preferences file, Read and understand the following link at the Lightroom Queen (better than similar at Adobe) Be ware of the consequences (mostly to settings you may have made in preferences)

https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/