Hello. I have just tried updating 2x Joomla 4.4.4 sites to 5.1, both running JA Coach template and near identical extensions loaded. One worked perfectly. The other gave a 500 error on front end. Back end working OK. The top level error message said "0 - Class "Joomla\CMS\MVC\View_JHtmlView" not found".
I turned on full error reporting and can send you the full error logs privately. I think it may be to do with T4.
I have since installed the last backup and the site, back to Joomla 4.4.4, is back working OK.
I would like to still upgrade to 5.1 and hope you can assist.
Thanks.