By Adam Nagy
1. appload command and load TestDll.ARX
2. close appload dialog
3. appload command and unload TestDll.ARX
4. close appload dialog
5. appload command and load TestDll.ARX
6. close appload dialog
7. appload command and unload TestDll.ARX -> AutoCAD crashes
I can see in your project's settings that you are using CLR (i.e. Common Language Runtime Support (/clr)).
This makes your project managed, which means that its extension should be *.dll and it should be loaded using NETLOAD, not APPLOAD.
Note that managed AddIns cannot be unloaded/reloaded in the same AutoCAD session.