If you use your storage configurator to break one or more mirror relationships, the server sees both production and backup SDs as primary. Not knowing which SDs to use, it fails the span.
On most systems, this behaviour is desirable. However, on certain complex systems such as the one described above, where the 'backup' SDs belong to another cluster and can legitimately be primary, the main cluster must not fail the span. Use the span-deny-access --secondaries command to make the server ignore backup SDs even if they become primary. It also makes it impossible for the server to fail over to those backup SDs.