I recently updated my Moodle installation to 3.8.1, then installed the Custom Certificate plugin. When I try to use the Upload Image, Manage Templates, etc. links on the Custom Certificates Setting page, it says the files are not found, though I can clearly see them in the cPanel file manager. I suspect the issue lies with these messages (the first one was cleared up before installation):
but wanted to verify that there's not an unrelated issue.