You need to create two different *.dll targets with the same Qt package (I have tested it with Qt 5.14.1 and Qt 5.14.2). Load Target_A and all is fine. Now load Target_B and you will end up in qErrnoWarning () in file "qwindowscontext.cpp" in method "QWindowsContext::registerWindowClass".
ATOM atom = RegisterClassEx(&wc);

The problem is that Target_B needs to register WNDCLASSEX which are already registered form Target_A. It has worked in previous version (Qt 5.13).

Here is the problematic bug/source
const bool classExists = GetClassInfo(appInstance, reinterpret_cast<LPCWSTR>(cname.utf16()), &wcinfo) == TRUE && wcinfo.lpfnWndProc != proc;

GetClassInfo () will return a BOOL. And the source is assuming it is fine with TRUE. But it is not. MSDN tells us
"If the function finds a matching class and successfully copies the data, the return value is nonzero."
In fact the result code is something like 4050. Which is nonzero. A BOOL is defined as an int. TRUE will not help here. The fix is 

const bool classExists = GetClassInfo(appInstance, reinterpret_cast<LPCWSTR>(cname.utf16()), &wcinfo) > 0 && wcinfo.lpfnWndProc != proc;

"> 0" instead of "== TRUE"

I hope that helps.