fNLO: compiling with root6.22 and gcc10

Dear experts

I want to run dijet production in fixed NLO mode, I succeed in producing histograms with Hwu format. Now I want to switch to root for having more options in histogram booking.
I use version MG5_aMC_v2_9_3 and the compiler setting with
source /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/ROOT-env.sh

However, using the template ananlysis code I get compiler errors, and I couldn't find where to change the g++ option (exporting CXXFLAGSS='-std=c++17 -lstdc++ ' does not help).
The compiler errors are:
-------------------------------------
INFO: Compiling P0_uxc_uxc...
WARNING: fct <function compile_dir at 0x7fb7f0cf2160> does not return 0. Stopping the code in a clean way. The error was:
A compilation Error occurs when trying to compile /nfs/dust/cms/user/jung/cvs/madgraph/MG5_aMC_v2_9_3/jj-13TeV-ptcut40-fNLO-NNPDF31NNLO-HT-root-req_acc_FO=0.1-test/SubProcesses/P0_uxu_uux.
The compilation fails with the following output message:
    /cvmfs/sft.cern.ch/lcg/releases/gcc/10.3.0-f5826/x86_64-centos7/bin/gfortran -O -fno-automatic -ffixed-line-length-132 -c -I. -I../../FixedOrderAnalysis/ -I/cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include ../../FixedOrderAnalysis//analysis_root_template.f
    /cvmfs/sft.cern.ch/lcg/releases/gcc/10.3.0-f5826/x86_64-centos7/bin/gfortran -O -fno-automatic -ffixed-line-length-132 -c -I. -I../../FixedOrderAnalysis/ -I/cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include ../../FixedOrderAnalysis//rbook_fe8.f
    /cvmfs/sft.cern.ch/lcg/releases/gcc/10.3.0-f5826/x86_64-centos7/bin/g++ -O -static-libstdc++ -std=c++11 -I/nfs/dust/cms/user/jung/cvs/madgraph/MG5_aMC_v2_9_3/HEPTools/lhapdf6_py3/include -c -I. -I../../FixedOrderAnalysis/ -I/cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include ../../FixedOrderAnalysis//rbook_be8.cc
    In file included from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TString.h:28,
                     from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TNamed.h:26,
                     from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TAxis.h:24,
                     from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TH1.h:24,
                     from ../../FixedOrderAnalysis//rbook_be8.cc:19:
    /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/ROOT/RStringView.hxx:84:17: error: expected type-specifier
       84 | operator std::string_view() const { return std::string_view(fData,fLength); }
          | ^~~
    In file included from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TNamed.h:26,
                     from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TAxis.h:24,
                     from /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include/TH1.h:24,
                     from ../../FixedOrderAnalysis//rbook_be8.cc:19:
.....
-----------------------------------------

In Template/NLO/Cards/FO_analyse_card.dat I have:
FO_ANALYSIS_FORMAT = root
FO_EXTRALIBS = Core Hist Matrix MathCore RIO dl Thread m
FO_EXTRAPATHS = /cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/lib
FO_INCLUDEPATHS =/cvmfs/sft.cern.ch/lcg/releases/LCG_99/ROOT/v6.22.06/x86_64-centos7-gcc10-opt/include
FO_ANALYSE = analysis_root_template.o

How could I get the proper root compiler options in place ?

thanks a lot in advance

Best
Hannes