Bear in mind that all of this is with Qt Creator (5.6.1 and 5.7):
I have a project made in my work machine (Win7, 64bits, compiled with MinGW 5.3.0 32bits in Qt 5.6.1). It works perfectly both in debug and release mode.
Soon after, I decided to test my program in another machine (WinXp, 32bits). I just passed the executable and the .dll. Still working like a charm.
Later, i tested the SAME version of my program in my gaming machine (Win10, 64bits). Weirdly, it didn't work at all. I received the following error:
Honestly, I don't know. But it very much seems like "Alexander Saprykin" answer is correct. During the process of testing, i had to update my Qt so it could fairly well be only a version mismatch.
Also, the fact that when I redid my build using only Qt 5.7.0 it worked, implies that it was indeed only a mismatch case.
As told by "Sebastian Lange" in the comments, using
windeplyqt.exe did the job. I just deleted everything but the
.exe in my build folder, openned Qt5.7 command line, chaged directory to the same as my
.exe and ran the command
windeployqt . or
Thanks to everyone!