A strange bug when transferring a program and changing the Qt version, how to catch it?

Question

Qt

Good time.
A very strange bug started happening and I don't know how to check.
The program was written in Qt 4.8, loaded a third-party library.
It worked for a long time, everything is ok.
Switched to Qt 5.4
The program began to crash at startup, it was precisely established that it crashes in the work of the connected library.
But I didn’t change anything about working with the library (neither directly nor indirectly, I just updated it for a start) when switching to a new version of Qt I didn’t change it!
Debugging is complicated by the fact that this is a library for working with third-party equipment and the connection is only remote.
How can you find out why the behavior has changed?
By not installing the development environment on the target system and, if possible, by not installing anything there at all, i.e. standard means of Qt or OSes?
Those. remote debugging.
The target operating system is Win 7 Embeded
SSH.

Answer 1

How do you load a third party library? Dynamic loading or registered in imports?