Despite there being a workaround for this problem (and thankyou, c-w-s and others, for detailing so clearly the steps that need to be taken and in which order), the fact remains that the average Joe Traveler who, quite reasonably, follows the instructions sent to them by BA stands a very high risk of failure and being unable to check in online. That is a really poor show from BA.

In my case, even the instructions in this forum didn't succeed, and I was stuck in limbo with emails telling me that everything had been verified and I was all set for OLCI, and MMB telling me that it wasn't.