-9136 error messages can be caused by a few issues in OS X with Mbox:
You may have added more RTAS plug-ins than your Pro Tools system can handle. Pro Tools LE systems use the host processor in the computer to process RTAS plug-ins. At some point, the host processor will be maxed out in it’s duties and throw a -9136 error message. Remove a plug-in or two to resolve this. Keep in mind 3rd party plug-ins use a lot of processing power and you may not be able to use too many of them (including software synths).
Recording to your system hard drive can bog down system performance and cause a -9136 error message. It’s ALWAYS better to record to a secondary hard drive. When you launch your Pro Tools sessions, save the session to a different (supported) hard drive other than your system drive. This is recommended for all Pro Tools systems, but is more highly recommended for Mbox systems for a couple of reasons. First, many Mbox users run their Pro Tools systems on an Apple laptop or iMac. These system hard drives are slower than 7,200 RPMs, which is required for recording a Pro Tools session. A supported firewire drive would be ideal in this case. Secondly, slower processor systems are greatly affected when recording to the system drive. There is always better performance when recording to a second hard drive.
Raise the hardware buffer size above 256 samples during recording to help prevent -9136 errors. (Setups menu > Playback Engine).
Playing a QuickTime DV movie via a Canopus converter can throw a -9136 error message over time. This is a known issue and has not been resolved.
Sometimes updating the firmware (or re-initializing the firmware) on Mbox can fix an unresolvable -9136 error message. Mbox firmware downloads are linked from this page:
Firmware version info for Mbox series (Mbox, Mbox 2, Mbox 2 mini, Mbox 2 Pro)
An unresolved -9136 error message can sometimes mean hardware failure and Mbox would need to be repaired. Call Digidesign Tech Support for an RA.