iOS/macOS Calendar (Nextcloud) "Cannot Connect Using SSL"

@itsthejb That version of openssl does not handle the current default chain from Lets Encrypt. The chain you send is fine and is the same as used by this website. It is only your openssl that is a problem.

Version 3.3.5 seems to fix it but I do not know anything about installing it on your system

See more info from openssl (not your version but good explanation)

And explanation of the current chain options