I am still facing the issue which probably related with Magento 2 (2.4.1) and Elastic search.
You can see the error page here: http://mage2.kouzloloutek.cz/loutky-spejbl-hurvinek
I read in another discussion thread on this site that it needs to be changed the field (in Magento admin) "Elasticsearch Index Prefix" from default "magento2" to "magento24", but the error page is still appears.
Attached are two pics. When the prefix was "magento2" and second one when it was "magento24". The error still persists. Please, give me an advice. I'm already desperate. Thank you very much.