documentclass[pdftex]{report}

usepackage{lipsum}
usepackage[many]{tcolorbox}    % Paket für textboxen
usepackage{varwidth}           % Paket zur anpassung der titelbreite von textboxen

begin{document}

newtcolorbox{mybox}{
    enhanced,
    before skip=2mm,after skip=2mm,
    colback=black!5,colframe=black!50,boxrule=0.2mm,
    attach boxed title to top left={xshift=1cm,yshift*=1mm-tcboxedtitleheight},
    varwidth boxed title*=-3cm,
    boxed title style={
        frame code={
            path[fill=tcbcol@back!30!black]
                ([yshift=-1mm,xshift=-1mm]frame.north west)
                arc[start angle=0,end angle=180,radius=1mm]
                ([yshift=-1mm,xshift=1mm]frame.north east)
                arc[start angle=180,end angle=0,radius=1mm];
            path[left color=tcbcol@back!60!black,right color=tcbcol@back!60!black,
                middle color=tcbcol@back!80!black]
                ([xshift=-2mm]frame.north west) -- ([xshift=2mm]frame.north east)
                [rounded corners=1mm]-- ([xshift=1mm,yshift=-1mm]frame.north east)
                -- (frame.south east) -- (frame.south west)
                -- ([xshift=-1mm,yshift=-1mm]frame.north west)
                [sharp corners]-- cycle;
        },
        interior engine=empty,
    },
    fonttitle=bfseries,
    title={My title},
    colbacktitle=green
}
begin{mybox}
lipsum[1]
end{mybox}

end{document}

boxed title style={
    frame code={
        path[fill=tcbcol@back!30!black]
            ([yshift=-1mm,xshift=-1mm]frame.north west)
            arc[start angle=0,end angle=180,radius=1mm]
            ([yshift=-1mm,xshift=1mm]frame.north east)
            arc[start angle=180,end angle=0,radius=1mm];
        path[left color=tcbcol@back!60!black,right color=tcbcol@back!60!black,
            middle color=tcbcol@back!80!black]
            ([xshift=-2mm]frame.north west) -- ([xshift=2mm]frame.north east)
            [rounded corners=1mm]-- ([xshift=1mm,yshift=-1mm]frame.north east)
            -- (frame.south east) -- (frame.south west)
            -- ([xshift=-1mm,yshift=-1mm]frame.north west)
            [sharp corners]-- cycle;
    },
    interior engine=empty,
},

Undefined control sequence. [   begin{mybox}]
Undefined control sequence. [   begin{mybox}]