I don't know how or what I did, but I don't have that plugin error anymore.
Anyway, I did again a release. I've put these files:
files.jpg
and when I run it, the "magic" white rectangle appears.
I tried again to run it from cmd, as you suggested, Chris, and here is the result:
app_running.jpg
and again Dependency Walker:
dependency_error.jpg

That MPR.dll seems to be inside Qt5Core.dll. What's wrong with Qt5Core.dll? Is there a problem with the file on Windows XP?