So solved this problem on my own. I am documenting the solution here in case it helps anyone else with troubleshooting.
Turns out it was simply a case of the LDAP DC IP address being incorrect. It was incorrect presumably because of restoring a backup clone of the VM and the IP address being re-assigned–not knowledgeable enough to understand why.
The reason the other accounts were all working is because years ago when I first set up this machine (without a clue that LDAP existed), I created the other accounts directly in Owncloud.
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