Thanks for the very fast reply.
What an excellent and fast answer -- more than I expected.

I myself thought now of changeing the make file to reduce the GCC 11 to check and compile with lower/older standards with the parameter -std=gnu+14 in CFLAGS and CXXFLAGS.

after "make clean"
and
make
I got errors as follows:
g++ -c -std=gnu++14 -pipe -O2 -Wall -Wextra -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_SERIALPORT_LIB -DQT_CORE_LIB -I. -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/include/x86_64-linux-gnu/qt5/QtWidgets -I/usr/include/x86_64-linux-gnu/qt5/QtGui -I/usr/include/x86_64-linux-gnu/qt5/QtNetwork -I/usr/include/x86_64-linux-gnu/qt5/QtSerialPort -I/usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o Config.o Config.cpp
Config.cpp: In function ‘QVariant getAX25Param(const char*, QVariant)’:
Config.cpp:61:16: error: ‘settings’ was not declared in this scope; did you mean ‘QSettings’?
?? 61 |???????? return settings->value(fullKey, Default);
????? |??????????????? ^~~~~~~~
????? |??????????????? QSettings
Config.cpp: In function ‘void saveAX25Param(const char*, QVariant)’:
Config.cpp:281:9: error: ‘settings’ was not declared in this scope; did you mean ‘QSettings’?
? 281 |???????? settings->setValue(fullKey, Value);
????? |???????? ^~~~~~~~
????? |???????? QSettings
make: *** [Makefile:527: Config.o] Fehler 1
-----------------------------------

any idea? any helpfull advices?

Thanks in advance
Michael