Aggregate (Apogee/SD-MixPre) giving 6101 error

[hagstrom71]

My main interface is an Apogee Symphony (thunderbolt) that works smoothly with PT 2019.5 and most previous PT versions I've used.

I've setup an aggregate to include a Sound Devices MixPre-10ii in addition to the Apogee for more inputs and I keep getting 6101 errors a few minutes in. I've tried unselecting/selecting ignore errors as well as every other configuration i can think of with the playback engine and it keeps crashing the record and giving the ol' 6101 CPU overload error. There are no plugins inserted.
This does not happen when using just the Apogee or just the MixPre, only when both are used.
Anyone run into this using an aggregate and multiple interfaces or have any suggestions?