Hello,
I had the same problem, on OVH too.
On the hosting of my customer, PHP was in 8.4 (for a CMS already installed)
As I have an other NextCloud instance on my own hosting provider (Infomaniak), I’ve tried to force install of v30.0.0, but it was not possible (PHP 8.3 max).
So I’ve found a solution by creating a .ovhconfig file in the folder where source files for NextCloud are stored.
In this file, I’ve let the value “app.engine=php” (and not phpcgi like suggested in an other topic : Nextcloud Displays White Screen on Desktop After Upgrade - #9 by Ulysse26 )
And I’ve only specified the PHP version in line “app.engine.version=8.3” instead of default “app.engine.version=8.4”
Now I’ve the CMS with PHP 8.4
And NextCloud with PHP 8.3
And no more “black and white unusable GUI” of NextCloud !