I've ignored the caldav and carddav warnings. You can use the url's in the relevant page within nextcloud user pages. In the panel on the left hand side of each relevant page, ie calendar url is on the calendar page, etc.
The database errors can be resolved from the command line. In the wiki entry for nextcloud on contribs.org there is instructions for setting maintenance mode, using those as a guide, substitute the commands for the database index rebuild. Info is on the link on the warning page to the nextcloud website with the commands or it's on the warning page (I frget which) but it's pretty easy to find.