As reported on Michael Tsai’s excellent site and elsewhere, if you’re on an older version of Mac OS X, you’ll encounter SSL errors due to a Let’s Encrypt root certificate that expired September 30, 2021. This includes communicating with Arq Cloud Storage servers like cloud.arqbackup.com. Arq may report this error as “The operation couldn’t be completed. (NSURLErrorDomain error -1202.)”.
The fix for this is to install an updated Let’s Encrypt root certificate on your Mac. Here are the steps, based on comments on Michael Tsai’s blog post:
Download this root certificate, using a computer that’s not experiencing this issue: https://letsencrypt.org/certs/isrgrootx1.der
Verify the fingerprint of the file you just downloaded by running this command in Terminal:
openssl sha256 isrgrootx1.der
The hash in the command’s output must match the hash shown on Apple’s support page (search for “ISRG Root X1”) to ensure you’ve got the correct file contents.
Copy the file to the affected Mac.
On the affected Mac, open the Keychain Access app (in Applications -> Utilites).
Pick File -> Import Items… from the menu, select your isrgrootx1.der file, change the “Destination Keychain” to “system”, and click “Open”.
In Keychain Access, click on “System” on the left (under “Keychains”) and double-click the “ISRG Root X1” item in the list. You should see a new window with an error “This root certificate is not trusted”.
Change “When using this certificate” to “Always Trust” and close the window. You’ll be prompted to authenticate as an administrator to save the change.
Reboot your Mac.
Open Safari and connect to https://cloud.arqbackup.com to verify that the SSL certificate is working as intended.