Package install worked yesterday, but not today. Very strange error reported

We are creating a managed package, which recently passed the received security review, and have had no issues installing it in DE and EE orgs…until today. Today, installs are failing in production and sandbox EE and Perf. Edition orgs with the following hard-to-comprehend error message.

Managed package can’t access class defined in subscriber org via Type.forName

I’m in the process of making a managed package extensible via an Apex plugins pattern. Basically there is an interface defined in the managed package that the subscriber Orgs can implement and then provide the class name to the managed package via a custom setting. The managed package creates an instance of the subscriber orgs class, casts it to the interface and then invokes the required interface methods.