Sorry for resurrecting this, but in case anyone stumbles over the same issue I would like to help out. The issue was in fact not related to my application finding the plugin DLL (open62541_backend.dll), but finding another DLL which the plugin relies on. On my development machine I fixed the problem by adding Qt/bin and QtCreator/bin paths to my PATH environment variable (a bad idea in my opintion but it did the job).
However, while deploying I stumbled over this issue again, with the root cause being a bit harder to find this time. After some digging it turned out that the plugin relies on msvcr100.dll which is part of Microsoft Visual C++ 2010 Service Pack 1. So installing this (very old package) or shipping the dll as part of the application will fix the issue. The dll can be found in the system32 folder, not within the compiler directories.
For anyone else coming across a similar problem, tools like dependency walker and procmon will your best friend. Procmon showed me that once open62541_backend.dll was loaded, the application tried to find msvcr100.dll in many different locations without success.
Don't copy these DLLs, unless it has changed over time, Qt Creator is build with MSVC and you have a MinGW Qt build. You should rather get the official OpenSSL package and install that one to use with your application.
I know this question is old, but it seems noone is answering BLE related questions here. (I've just posted such a question myself, and fear noone will anwer it either).
I developed an app based on the same example, and it worked fine under both android 4.3.2, 5.1.2, and 6.0. However, the customer came with an android 5.01, and sure enough, the app did not work as expected. It seemed to "hang" and tried to connect to two devices at the same time, etc. We did not investigate it further. This was with Qt 5.6, so the issue is not resolved, and may never be, since with newer versions of Android it is working fine.