If you are updating Lemmy to version 0.18.0 you may want to be aware of this bug that exists with site icons. It will break your site.
0.18.1 exists
As far as I know the bug still exists. It is caused by having the host-name of your server set to the same thing as the domain name for your Lemmy instance. The quick and easy fix is to change the host-name of your server.