It is even stranger than I thought. I am probably making a mental error somewhere.

The header of the class in the plugin reads
class remote: public rigInterface, public Ui_Form {
Q_OBJECT
Q_INTERFACES (rigInterface)
#if QT_VERSION >= 0x050000
Q_PLUGIN_METADATA (IID "remote")
#endif

This does not work, the Meta data is apparently not found while running.
If I just use
class remote: public rigInterface, public Ui_Form {
Q_OBJECT
Q_INTERFACES (rigInterface)
Q_PLUGIN_METADATA (IID "remote")

It works fine.

Now I checked, and the test
#if QT_VERSION >= 0x050000
evaluates to true, so although I am able to circumvent the error, I am lost.

As sais before, on other systems (also testing on an old Ubuntu box) it compiles and works fine, both using Qt4 and Qt5.
JanK