There is no error when calling (use-external-katex...) . It is just that katex folder is still created locally, and the local .html doesn’t update the calling references to use the external-katex folder.

Sorry, because I won’t be able to keep checking the advances. For the moment I will do the changes manually, in order not to have the katex folder in each of the local documents. Thanks again for the maintenance and future improvements to this important package. Please Keep Safe. Best regards.