-There are an increasing number of plug-ins that will only instantiate on a specific DSP chip. However, the DSP chips on MIX cards do not all have access to the same amount of RAM:

-3 chips have SRAM (Static Random Access Memory). Extremely fast but limited in size. Good for reverbs and short delays (this chip is required for plug-ins such as MegaVerb, LexiVerb and many third party plug-ins).

-2 chips have DRAM (Dynamic Random Access Memory). Slow but large in size. Good for long delay
effects.

-1 chip has no external RAM.

-The DSP's on the MIX cards have many times more internal memory than a 56002 (i.e. chips used on the DSP Farms), so there is enough to do many processes including EQ, dynamics, and mixing.

-This begs the question... why not build a card with all SRAM chips instead of SRAM, DRAM and no RAM together? The original DSP Farms all had SRAM. This was wasteful when it came to doing long delays liken Procrastinator (1 per DSP limited by RAM) and EQs (which didn't use very much SRAM). Bottom line, depending on the type of plug-ins you use (i.e. whether or not the plug-ins are mult-shell), you will be able to get more plug-ins using one Mix Farm than two DSP Farms.

-An analysis of RAM usage showed that we could best use available board real estate and power by giving different flavors of DSP. This allows more instances per chip, as well as longer delays. The tradeoff is that you can run out of "compatible" chips while there are still DSPs available.

If compatible chips are available:
-Trash the DigiSetup File, Pro Tools preference, and DAE prefs (the entire folder) from the System Folder>Preferences folder. Empty the Trash and restart the Mac. When you go back into Pro Tools, do so from the Pro Tools Application Icon and not by double clicking on a session. Once Pro Tools is launched, reconfigure the hardware settings.

-You may have a corrupt session. Create a new session and move the tracks using the File>Import Tracks command and see if the error persists.