For increase visibility and because i was affected by the same issue, I allow myself to reference the solution from the comments in a dedicated answer:

.NET 9 enables Control-flow Enforcement Technology (CET) by default for managed processes. Visual Studio’s current debugger has compatibility issues with CET, causing the fatal error. Workaround: Disable CET for your app by adding this to your .csproj:

<PropertyGroup>  
   <CetCompat>false</CetCompat>
</PropertyGroup>

This allows debugging to work again.

All credits to @Chad Decker for sharing his solution!