Hi this seems to be same issue as reported by Luc earlier on this:
> On 13.07.21 00:40, jfbu wrote:
>> pip install -U sphinxcontrib.serializinghtml
>>> and perhaps same with sphinxcontrib.htmlhelp as I see I updated both
>>> on June 13
> Yes, this fixed my problems. Great! Thanks!
Try thus (I don't recall the conda syntax) to use conda to update both of sphinxcontrib.serializinghtml and sphinxcontrib.htmlhelp