Hi @Salt&Pepper
Thank you for posting your question in the Microsoft Q&A forum.
We're sorry to hear you're encountering this issue when installing the latest CU patches.
As moderators, we don’t have access to an on-premises environment to replicate this scenario, since our testing capabilities are limited to SharePoint Online. So, I’ve done some research to help guide you in the right direction.
On SharePoint Server Subscription Edition (SE), the modern client assets are deployed under: C:\Program Files\Common Files\microsoft shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\Next\spclient
On your SharePoint server, navigate to the folder above and check whether subfolders exist (for example, locale folders like en-us or default) containing .js files such as sp-pages-assembly.js.
If the folder is missing, ensure you’ve installed both the STS update and the WSSLOC (language-dependent) update for your CU. Missing WSSLOC often breaks modern UI. If no WSSLOC was released for the month, install the latest available one.
You can read more at SharePoint updates - Office release notes | Microsoft Learn.
If the issue persists, please consider running the configuration wizard again, as this often resolves many common SharePoint Server issues
I hope this helps resolve the issue.
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