This can occur in Pro Tools 6.7 and higher if logged in as a non-admin user on a session that was created on another system, when doing any operation that requires that the session be closed and reopened such as declaring Legacy peripherals, enabling Auto Delay Compensation, changing the DAE Playback Engine, changing the number of voices and DSP's, etc.
The workaround is to re-save the session on the current system, and then perform the desired operation.