Guys, problem solved. Everything is working.
I didn’t have to do anything. The cPanel license from the hosting provider has expired. After regularization, NextCloud went back to work.
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Since I found this thread from a search trying to fix what seemed to be an identical problem, with the same error page, I thought I would document my own resolution. Long story short–like above, I was tearing my hair out looking at the Apache log instead of the Nextcloud log–it turned out that the LDAP server being used for authentication for some users was not reachable from the Nextcloud server. Once the LDAP server was back up, Nextcloud behaved appropriately.