On a UCM system I have worked on, we were running into problems with both Music on Hold and ad-hoc conferencing when a recording profile was applied to DNs. Music on Hold would not play, and conferences could not be initiated. The problem turned out to be codec related: parts of the call were G.722, parts G.711. Once the recording starts, it locks the codec for the call. If MoH or a conference tries to renegotiate it, you get the behavior we experienced.
To correct this, we chose the option to disable G.722 on the phones that were set to be recorded, so there was no need for codec renegotiation. You could also disable G.722 on the cluster, but the customer did not want to take that option.