FWIW I also experienced that problem recently, albeit in a completely different context. I was unable to get the ClassicPress SSL Fix plugin to work even after enabling the 3-minute insecure requests, and I, similarly, had been with the host for 9 years. Because I left the host after the incident, I never got to the bottom of it, but it seems the host itself was behind the “SSL certificate problem: certificate has expired.” FWIW it was the only time I tried to use the migration plugin after my original migration to CP, and the only time I ever used the ClassicPress SSL Fix plugin.