C drive getting filled, while lightroom is not even installed there

3. System Info

Looks fine, nothing stands out. Verifies OS, LrC, and GPU info as current, up to date.

2. LrC catalog backup location, Future problem

If I see the info correctly, you are placing the LrC catalog backup on your M drive. So not the C drive.

Problem is, your actual catalog is on the same hard drive. When that hard drive fails and you cannot access your catalog, when you need the backup, the backup will also be gone. Backups should be to separate hard drives than the material they are backups of.

1. Camera RAW CACHE, maximum size.

Your screenshot show it at the default value of 5GB, way too small foe even small catalogs. LrC might be reaching out to other resources as a result during develop mods. Try something more lime 70GB

2. Camera RAW CACHE location. (not the issue)

Fine, not on C. I had a suspicion it would be on C drive, glad to see that it us not. LrC can leverage hard drive speed for catalog location, and Camera RAW CACHE location. And on the flip side, for Windows OS, Camera RAW CACHE on the C drive can be a liability as it will compete fir hard drive read/wrights with the Paging file. Not the case here.

Logged in on Edge now, rather than Chrome, here I CAN reply. 

Lightroom Classic-versie: 10.4 [ 202108071231-af9219b9 ]
Licentie: Creative Cloud
Taalinstelling: nl
Besturingssysteem: Windows 10
Versie: 10.0.19042
Toepassingsarchitectuur: x64
Systeemarchitectuur: x64
Aantal logische processors: 8
Processorsnelheid: 4,0GHz
SQLite-versie: 3.34.0
Ingebouwd geheugen: 16209,3 MB
Beschikbaar fysiek geheugen voor Lightroom: 16209,3 MB
Fysiek geheugen gebruikt door Lightroom: 3862,9 MB (23,8%)
Virtueel geheugen gebruikt door Lightroom: 14502,1 MB
Aantal GDI-objecten: 893
Aantal gebruikersobjecten: 2737
Aantal proceshandelingen: 2727
Cachegeheugengrootte: 1146,5MB
Interne Camera Raw-versie: 13.4 [ 872 ]
Maximumaantal door Camera Raw gebruikte threads: 5
Camera Raw SIMD-optimalisatie: SSE2,AVX,AVX2
Virtueel geheugen Camera Raw: 2472MB / 8104MB (30%)
Fysiek geheugen Camera Raw: 2484MB / 16209MB (15%)
Dpi-instelling systeem: 96 dpi
Bureaubladcompositie ingeschakeld: Ja
Schermen: 1) 2560x1440, 2) 1920x1080
Invoertypen: Multi-aanraking: Nee, Geïntegreerde aanraking: Nee, Geïntegreerde pen: Nee, Externe aanraking: Nee, Externe pen: Ja, Toetsenbord: Nee

GPU-info:
DirectX: NVIDIA GeForce GTX 1060 6GB (30.0.14.7111)

Toepassingsmap: M:\Adobe\Adobe Lightroom Classic
Bibliotheekpad: M:\Extra Afbeeldingen\Fotos D7500 Raw\2021 - Zomer\2021 - IJsland\2021 IJsland\2021 IJsland.lrcat
Map Instellingen: C:\Users\Max\AppData\Roaming\Adobe\Lightroom

Geïnstalleerde plug-ins:
1) AdobeStock
2) Facebook
3) Flickr
4) Tetheringplug-in voor Nikon

Just a test on the reply button.

Yes, very annoying that the reply button only exist in the original post, not in a response/reply.

Button works, this from Safari on an iPad.

I have noticed oddities in the site, as if the server is having issues, or perhaps traffic issues.

And in Catalog Settings, where is your backup being kept (if at all)

Also, can you post a screenshot from LrC, a screenshot of /Preferences/Performance/

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut as that is just so much dead space to us non-Techs.

/edit/ content originaly below in this reply deleted as to not clutter up discussion, now that example is no longer called for.