SageMath Tutorials not working inside JupyterLab

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Summary: When I select Help for sagemath from the menu in Jupyterlab, the browser throws "allow-same-origin" errors in the console and eventually stops processing pages completely, showing "math processing error" for all formulas.

I am trying to add SageMath to our existing JupyterLab 1.2.6 installation inside a docker with an Ubuntu 18.04 image. I installed SageMath 9.0 and added SageMath to Jupyter via jupyter kernelspec install ./local/share/jupyter/kernels/sagemath.

Everything seems to run fine, except for the help:

• I select "Sage Documentation" from the Help menu in a SageMath notebook
• In the Web Browser console I get The document is sandboxed and lacks the allow-same-origin flag.
• While the document seems to be working anyway, going through the doucmentation yields an error for every single formula it encounters, specifically: Access to font at XX (mathjax-font) from origin null has been blocked by CORS policy, No Access-Control-Allow-Origin header is present on the requested resource.
• Still, it seems to be working somewhat, I think with the wrong fonts, but readable.
• At some point MathJax stops processing formulas completely and shows Math Processing Error for each formula.

I already tried setting allow-origin in the notebook configuration, but that did not change anything.

Anybody has a clue?

Frank