this is not a request for help, but rather a report after some issues installing Nextcloud 18 on a shared hosting (OVH)
First of all a suggestion for those who want to use https (maybe obvious for many): if you plan to use https, it’s better to first set up the ssl cert for the hosting and install NC when it’s up and running, rather than adding https support later. In this way you don’t have to change config and everything works more smoothly.
In my case, I have a shared hosting on OVH and had some issues for missing icons and server errors.
Everything is OK now, after changing the settings to use phpcgi instead of php and disabling the application firewall.