I managed to narrow the 1500 line long SVG to a short snippet that can be used to reproduce the error:
<svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0" y="0" height="100%" viewBox="-0.1964024156332016 -0.0687660425901413 1000.392822265625 350.1374816894531" width="100%" preserveAspectRatio="xMidYMid meet">
<defs id="svgEditorDefs">
<marker id="arrow10-8-right" markerHeight="8" markerUnits="strokeWidth" markerWidth="10" orient="auto" refX="-3" refY="0" viewBox="-15 -5 20 20">
<path d="M -15 -5 L 0 0 L -15 5 z" fill="black"/>
</marker>
</defs>
<g id="g_e1_svg" style="">
<line id="e1_line" x1="831.3800242832729" y1="311.218994140625" x2="831.3800853184291" y2="326.8453369140625" style="stroke: red; fill: none; stroke-width: 2.97px; marker-mid: url("#arrow10-8-left");"/>
</g>
</svg>
Which results in this:
The problem is that the SVG string contains a lot of "
strings (probably by getting it from a third-party SVG editor)...
I assume that I have to remove those strings automatically, if available ...