iTunes can't verify the identity of the server

So I'm experiencing a problem which seems to be pretty common. When I open iTunes, a window pops up which says:

iTunes can’t verify the identity of the server “init.itunes.apple.com”.

The certificate for this server is invalid. You might be connecting to a server that is pretending to be “init.itunes.apple.com”, which could put your confidential information at risk. Would you like to connect to the server anyway?

It gives me the option to cancel or continue. If I cancel, the message disappears, and if I continue, identical messages pop up for the following servers:

“p17-buy.itunes.apple.com”

“xp.apple.com”

“upp.itunes.apple.com”

I've looked around and found a list of solutions to the problem, but nothing is of any help. I'm not using an old version of iTunes (I have version 12.1.2.27). I am on an administrator account. My date and time are set properly and the "set date and time automatically" box is checked. I have tried the following already:

-Checked and unchecked the option "check for new software updates automatically."

-Reset the iTunes store cache.

-Cleared cache on both Chrome and Safari, and deleted most of my extensions.

-Checked keychain access, but did not find any certificates with the blue plus sign.

-Dragged the file "NetworkInterfaces.plist" to the trash and restarted.

-Checked the hosts file via the terminal, but found no entries containing "apple.com."

-Deactivated firewall.

None of these had an effect. I can't update iTunes to fix the issue because I can't connect to the iTunes store or software update. Stranger still, when I try to access any page from apple.com, including this page and https://appleid.apple.com/, which is what the iTunes advanced troubleshooting page recommends you open as a "secure website to test if you are online,” I receive the following message:

Your connection is not private

Attackers might be trying to steal your information from [url] (for example, passwords, messages, or credit cards). NET::ERR_CERT_AUTHORITY_INVALID

Clicking “advanced” reveals another message:

This server could not prove that it is [url]; its security certificate is not trusted by your computer’s operating system. This may be caused by a misconfiguration or an attacker intercepting your connection.

I can then click a link to proceed to the page, but it won’t load properly (just text against a white background, no images or anything) and the “https” in the url is in red with a strikethrough. This appears to only happen with apple, as I can visit other https pages with no issue.

So basically I’ve exhausted every suggestion to no avail. Is there anything else I can do?