Some times there are .net applications that fail when QTP opens with .net add-in.
In order to confirm if the hooks of the .net add-in are affecting the application you can follow this steps to disable the hooking on the application and then check if it works normally again.
a. Launch the Registry Editor(Start>run type Regedit and press enter or click ok button)
b. Navigate to HKEY_CURRENT_USER\Software\Mercury Interactive\QuickTest Professional\MicTest\HooksMgr\DllInjector\ALWAYS\
c. Add a new key with the name of your application executable to the key. Right-click on "ALWAYS" and select "New -> Key". Enter the name of the application executable.
d. Add an "Enabled" value to the key. Right click the key you created and select "New -> DWORD Value". Enter Enabled as the name and 0 as the value
e. Add a "UseDefault" value to the key. Right click the key you created and select "New -> DWORD Value". Enter UseDefault as the name and 0 as the value.