In case of ActiveQt dll Platform plugin is never picked up from its deployed directory.

Deploying ActiveX Dll runs into problems with the Qt platform abstraction.
When an application loads the ActiveX-Dll (starting with regsvr32.exe for registering) the Qt library searches for the QWindows.dll (platform abstraction) and no matter where windows.dll is kept it never finds it.
-App\plugins\platforms
-App\platforms
-App

It can be easily reproduced with the Qt Example in examples\activeqt\wrapper.

Just build this example with MSVS2010 (it registers well after building on development machine) and deploy it to target machine where Qt is not present. Copy all needed Dlls and subdirectories into the directory where the AxtiveX (wrapper) Dll is located.

Then type "regsvr32 wrapperax.dll". (it will complain about windows QPA)

Only way to get it register is to set the QT_QPA_PLATFORM_PLUGIN_PATH in terminal.

It is may be related to QTBUG-29550