If you’ve ever seen this error, you know how frustrating it is. It took me about 3 days to work this one out, but here’s the solution.
For some reason, sometimes the httpd includes that Plesk spits out in /var/www/vhosts/<domain>/conf gets out of sync with Plesk, and you might end up with orphaned includes which refer to old IP addresses which are no longer on the system. These still get read by Apache on startup, and cause problems with the Plesk Migration Manager (PMM).
To fix, simply remove the unused/old include from the formentioned conf directory, and PMM should migrate the subscription just fine. You can easily tell which are orphaned by running a “ls -latr” and see which is the oldest.
The other thing to check is if the subscription has domain aliases which are out of sync with DNS on the main domain. This will probably refer to old IP addresses no longer on the system. Simply resync DNS to fix.