Could not create the Minuit2 minimizer

Thanks. That looks also correct, so we have to dig deeper.

Can you run this debug macro with root -b -q debug_macro.C and paste the output here? Something seems to be going wrong with the ROOT plugin system.

void debug_macro() {

    gDebug = 3;
    auto h = gROOT->GetPluginManager()->FindHandler("ROOT::Math::Minimizer", "Minuit2");
    h->LoadPlugin();
    auto * min = (ROOT::Math::Minimizer *)( h->ExecPlugin(1,"migrad" ) );
    std::cout << min << std::endl;
}