ZFS labels found on both partitions in the mirror.
No SMART errors have been logged.
All SMART selftests have passed.
The only other thing I could advise is to run a long selftest on both drives, to truly rule out a drive failure. Otherwise, I don’t know why you’re getting an I/O error on attempting to import.
Someone else with more knowledge might have other ideas.
Another thing you can try is to import the pool in Core 13.3, to see if it’s importable.
In the meantime, don’t do this. If you find something online, first ask in here if it’s safe to do.