Remember it is a wildcard cert. It goes to multiple servers. Just happens I made it for the untangle but it is the same format OMV wanted. You need to find the name to the name of the cert that you imported by web ui. Then rename it to that.
Sorry for the confusion.
If you make it idiot proof, somebody will build a better idiot.
I tested your method and my method again tonight and they both worked.
after import the cert and keys into OMV via the Import menu, go to /etc/ssl/private and /etc/ssl/certs, to find the openmediavault-unique-key....
Next, use cp to copy the files from /etc/letsencrypt/live/example.com/ to /etc/ssl/private and /etc/ssl/certs appropriately.