Microslop released a security update for Windows that seems to have knock-on effects for RDS session host servers [mainly] causing issues preventing users from logging on, and can even render the server console inaccessible.
Microslop has since released patches to address these issues, but must be manually downloaded from the catalog and installed.
This bug isn't limited to RDS session hosts and I have seen in the wild Domain Controllers and other non-RDS based servers also suffer from this issue.

Use PowerShell command: Get-HotFix -ComputerName "ServerName" to get a list of the installed updates & fixes
If you see any of the listed hotfixes below installed, then you need to apply the patch.
If none of the affected hotfixes are installed, then you may be able to hide the update from installation, but I've had mixed success with this and have had to resort to manually uninstalling the update then disabling Windows Update services until the update is revoked.

Affected HotFixes:
Windows OS:	KB:		Build:
--------------------------------------------
Server 2016	KB5123099	–
Server 2019	KB5122876	–
Server 2022	KB5122882	20348.5622
Server 2025	KB5122871	26100.33438
Windows 11	KB5124008	 
Windows 11	KB5120998	 

KB's to install that will patch the above hotfixes:
Windows OS, build:						KB Patch:	Catalog Link:
------------------------------------------------------------------------------------------------------------------------------------------------
Windows 11, version 26H1: 	-	-	-	-	KB5129194  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129194
Windows 11, versions 25H2 and 24H2: 	-	-	-	KB5129195  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129195
Windows 11, versions 25H2 and 24H2 (hotpatch): 	-	-	KB5129241  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129241
Windows 11, version 23H2: 	-	-	-	-	KB5129242  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129242
Windows Server 2025: 	-	-	-	-	-	KB5129235  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129235
Windows Server 2022: 	-	-	-	-	-	KB5129237  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129237
Windows 10, version 22H2 and Windows 10 Enterprise LTSC 2021: 	KB5129236  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129236
Windows 10 Enterprise LTSC 2019 and Windows Server 2019: 	KB5129238  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129238
Windows 10 Enterprise LTSB 2016 and Windows Server 2016: 	KB5129239  	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129239
Windows Server 2012 R2: 	-	-	-	-	KB5129243 	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129243
Windows Server 2012: 	-	-	-	-	-	KB5129244 	https://www.catalog.update.microsoft.com/Search.aspx?q=KB5129244

Patches are also available here on this server. These are direct copies of the above catalog files.

More information:
https://support.citrix.com/external/article/CTX697101/issues-with-microsoft-windows-september.html