Moving from QT 5.12.8 to 5.15.2 MOC fails - c++

Question

We try to mov to the latest Qt 5 LTSB (5.15.2) from our working base Qt 5.12.8 but ran into MOC errors:
The moc process failed to compile
   "SRC:GenericForm.h"
into
   "SRC:moc_GenericForm.cpp"
Output
   SRC:/workspace/NBCI2/src/base/base_language.(9): Parse error at "{"
The problem occours whith every Qt build greater 5.12.x what do we missing? What is changed on moc between 5.12.x and later?
Build system is cmake
base_language is include by some of the include headers.
GenericForm.h:
#pragma once
#include "host.h"
#include "qt_type_defines.h"
#include <qwidget.h>
#include "qt_in_scheduler.h"
#include "qt_out_scheduler.h"
#include "GenericBaseForm.h"
namespace qt
{
using namespace nbc;
class GenericForm : public GenericBaseForm
{
Q_OBJECT
public:
GenericForm(host::ihost * h, const GenericFormConfig & cfg, safe_bool & Shutdown, rmutex & form_creation_mtx);
virtual ~GenericForm();
virtual void SetTitleError(bool on);
virtual void SetTitle(str_cref title_str);
virtual void SetHintText(str_cref hint_str);
virtual bool AllowHide();
virtual void OnLanguageChange(lang_t)
};
}
base_language:
#ifndef BASE_LANGAUAGE_H
#define BASE_LANGAUAGE_H
#include "base.h"
namespace base
{
enum struct lang_t : char
{
default = -1,
english = 0,
german = 1,
french = 2,
};
base::str_t to_string(lang_t);
lang_t from_string(base::str_cref);
}
#endif
CMake.txt:
CMAKE_MINIMUM_REQUIRED(VERSION 3.17.0 FATAL_ERROR)
SET(VERSION_MAJOR 1)
SET(VERSION_MINOR 0)
SET(VERSION_PATCH 0)
SET(PROJECT_NAME "lib_GenericForms")
# find Qt5 and required components
FIND_PACKAGE(Qt5 COMPONENTS REQUIRED Core Gui Widgets )
#need this to include the auto generated ui_mainwindow.h
#file from the qt5_wrap_ui call below.
IF(CMAKE_VERSION VERSION_LESS "3.7.0")
SET(CMAKE_INCLUDE_CURRENT_DIR ON)
ENDIF()
INCLUDE_DIRECTORIES(
)
ADD_DEFINITIONS( -DBUILD_STATIC -DQT_SUPPORT_LIB
)
SET(CMAKE_AUTOMOC ON)
SET(CMAKE_AUTOUIC ON)
SET(CMAKE_AUTORCC ON)
SET(ENVIRONMENT_EXTENSION_DEBUG
${ENVIRONMENT_EXTENSION}
)
SET(ENVIRONMENT_EXTENSION_RELEASE
${ENVIRONMENT_EXTENSION}
)
#set project sources
SET(PROJECT_SOURCES
${CMAKE_CURRENT_LIST_DIR}/GenericBaseForm.h
${CMAKE_CURRENT_LIST_DIR}/GenericBaseForm.cpp
${CMAKE_CURRENT_LIST_DIR}/GenericForm.h
${CMAKE_CURRENT_LIST_DIR}/GenericForm.cpp
${CMAKE_CURRENT_LIST_DIR}/GenericSensorForm.h
${CMAKE_CURRENT_LIST_DIR}/GenericSensorForm.cpp
)
SET(PROJECT_RESOURCES
)
#set project sources
SET(PROJECT_UI
)
# setup project
ADD_LIBRARY(${PROJECT_NAME} STATIC
${PROJECT_SOURCES}
${UI_WRAP}
${MOC_SOURCES}
)
TARGET_LINK_LIBRARIES(
${PROJECT_NAME}
PUBLIC
Qt5::Core
Qt5::Gui
Qt5::Widgets
)
GenerateVSUserFile(
"${ENVIRONMENT_EXTENSION_DEBUG}"
"${ENVIRONMENT_EXTENSION_RELEASE}"
"$(TargetDir)"
"$(TargetDir)"
"$(TargetPath)"
"$(TargetPath)"
""
""
"${PROJECT_NAME}"
)
GenerateVersionInfo(
1
0
0
"${PROJECT_NAME}"
"${PROJECT_NAME}"
"${PROJECT_NAME}.lib"
"${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.rc"
"${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}_version.h"
)
# Executables fail to build with Qt 5 in the default configuration
# without -fPIE. We add that here.
set(CMAKE_CXX_FLAGS "${Qt5Widgets_EXECUTABLE_COMPILE_FLAGS}")
#Set the target link subsystem
SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /SUBSYSTEM:WINDOWS")
#set compiler flasgs
SET(COMPILER_FLAGS "${COMPILER_FLAGS} /MP")
SET(COMPILER_FLAGS "${COMPILER_FLAGS} /EHsc")
SET(COMPILER_FLAGS "${COMPILER_FLAGS} /Zc:wchar_t")
SET(COMPILER_FLAGS "${COMPILER_FLAGS} /wd4290")
SET_TARGET_PROPERTIES(${PROJECT_NAME} PROPERTIES COMPILE_FLAGS "${COMPILER_FLAGS}")
SET(PROJECT_NAME_INCLUDES ${CMAKE_CURRENT_LIST_DIR} PARENT_SCOPE)
SET(PROJECT_NAME_PROJECTNAME "${PROJECT_NAME}" PARENT_SCOPE)