SSL Error - Can't connect to server

After updating my 2019 iMac to Catalina 10.15.6, I'm having massive internet problems. Safari can't establish secure connections and does not open any websites, I can't check for updates because an SSL error prevents connection and none of the other internet-reliant apps (TV, iTunes, Maps etc.) work due to no connection.

Here is what I have done so far:

I checked and restarted my router, everything works fine for every other appliance but the iMac.

I changed my DNS, no change.

I changed the network location, no change.

I deleted my WiFi settings and then added them again, no change.

I set the certificates for the website I was trying to visit (in this case Apple.com) to "always trust", no change.

There are no programs installed that mess with my network, no VPN, no proxy.

I de-installed my virus scanner (Kaspersky) and even played around with de-activating the firewall to see if that changed anything. No luck.

In safe mode, I can connect and everything works just fine, but when I restart normally, no internet connection.

My MacBook Pro (which I prevented from updating automatically and is still running 10.15.5) works just fine.

Is this a certificate problem (I remember having to delete all my certificates a while back after another macOS update to solve a similar problem, but I don't remember exactly what I had to do to resolve the issue)?

Grateful for any helpful input!

Thanks!