probably best way would be leaving Hestia and returning to vesta or another panel.
sorry mate, but we can’t debug your system for you, that’s something you need to figure yourself.
not offense meant, but that’s what logfiles and stuff are for… first figure which upstream is gone away and why. could be apache, php-fpm or mysql.
you should not have to fiddle with the configs at all, I have nextcloud running without issues for quite some time and haven’t changed anything.
also could be specific to plugins/extensions that you use in nextcloud. or the workload, size of files etc. - we simply can’t know and that’s something you need to use your admin powers for to find out.
best of luck.