Zoom blank or green screen?

This should not be an issue on a M1 Ultra for sure. You should set the GPU to auto on these. Only issue I see here is that your standard preview size is set too low which might make the machine slow in browsing in Library. Set the satndard preview size to auto and it should select 2880 for the standard size. Don't think that's the issue but possible.  Also many people have found that you should enable HDR in Library in the preferences even if you don't use HDR at all for best performance. Not clear why.

Lastly, have you by any chance set the performance setting to use smart previews instead of originals by any chance? You probably shouldn't with this machine and smart previews are too small resolution for your monitor anyway.

Just tried turning off GPU acceleration - makes no difference at all. 

Library mode - Zoom = centre picture goes bright green
Develop mode - Zoom = picutre dissapears - just the grey background visible
Full Screen mode - Zoom = black screen

Mac Studio M1 Ultra

Haven't tried turning off Graphics Acceleration. But a bit sad if that has suddenly become an issue. Its normally set to Auto, but today I did try setting it manually, with all boxes ticked apart from HDR. 

Lightroom Classic version: 14.0.1 [ 202410161356-30922cfc ]
License: Creative Cloud
Language setting: en-GB
Operating system: Mac OS 15
Version: 15.1.0 [24B83]
Application architecture: arm64
Logical processor count: 20
Processor speed: NA
SqLite Version: 3.36.0
Built-in memory: 65,536.0 MB
Dedicated GPU memory used by Lightroom: 862.4MB / 49,152.0MB (1%)
Real memory available to Lightroom: 65,536.0 MB
Real memory used by Lightroom: 4,499.2 MB (6.8%)
Virtual memory used by Lightroom: 426,259.8 MB
Memory cache size: 17,447.1MB
Internal Camera Raw version: 17.0 [ 2043 ]
Maximum thread count used by Camera Raw: 5
Camera Raw SIMD optimization: SSE2
Camera Raw virtual memory: 69MB / 32767MB (0%)
Camera Raw real memory: 73MB / 65536MB (0%)

Cache1:
NT- RAM:0.0MB, VRAM:0.0MB, Combined:0.0MB

Cache2:
m:17,447.1MB, n:0.0MB

U-main: 144.0MB

Standard Preview Size: 1440 pixels
Displays: 1) 5120x2880

Graphics Processor Info:
Metal: Apple M1 Ultra
Init State: GPU for Export supported by default
User Preference: GPU for Export enabled
Enable HDR in Library: OFF

Application folder: /Applications/Adobe Lightroom Classic
Library Path: /Users/pt5/Library/Mobile Documents/com~apple~CloudDocs/Lightroom Cloud/240831 Annie & James.lrcat
Settings Folder: /Users/pt5/Library/Application Support/Adobe/Lightroom

Installed Plugins:
1) AdobeStock
2) Aperture/iPhoto Importer Plug-in
3) DxO PureRAW 2
4) DxO PureRAW 2 Importer
5) Flickr
6) HDR Efex Pro 2
7) Luminar 4
😎 Topaz Photo AI

Config.lua flags:

Indeed looks like a GPU issue. What hardware do you have? Can you post a System Info (Help menu). If this is an older Mac with a AMD GPU that might be the issue.

And if in /preferences/performance/ you turn Off the use of the GPU?