Installation Issue

The errors are cosmetic. The problem is that your server is sending them to the browser instead of an error_log.

You can disable the errors, see Error from 2.4.8 to 3.0.2 - #2 by ajnyga

But it depends on the service provider. In some cases if you have cpanel, you can change the error settings from there. Using a htaccess file can also work, but not on all errors. You could also ask you service provider about the error settings and whether they could be changed for you.