Jea-Slider/gallery doesn't work in Jea after an update from joomla 3.3. to 3.8. I was trying the following as you said in Joomla 3.7. ( https://github.com/joomla/joomla-cms/pull/15600/files), but this did not help. I don't have such a html.php-File in libraries/cms/html/html.php. And if I create one, it doesn't make any difference. I have problem with upload pictures, they upload but disappear again after a short time when I go back to frontend. Gallery frontend process is hanging if I want to open a thumb-picture.
The following Language message in German backend also appears: WARNUNG: Die Funktionen wurden für die aktuelle Sprache reintialisiert. Bitte überprüfen Sie die hervorgehobenen Felder. It shows red field "slogan" as soon as I klick on a different language.
Has this something to do with image-gallery? I have installed Joomla languages French, German, Englisch.
Would be happy for any help.