That error means that IOS had trouble communicating with the DSP. DSP could have crashed, or just didn't respond for a little bit (and maybe gracefully recovered). Worst case scenario, call drops and no audio. Best case scenario, it doesn't effect calls and is a cosmetic error where the DSP recovers properly.
Always a bug when this error is seen (though if the issue is only ever on the same DSP, even after a reboot, it could be bad DSP hardware).
12.4(22)T should be running DSPware 23.8.x, not 9.4.x. If you are running 12.4(22)T with 9.4.x, that's not supported, will cause issues, and you should delete all .rbf and .dsp files from flash and reload the router so that you are running the supported DSPware for that IOS release.
FWIW, DSPware < 9.4.14 in that DSP throttle is suceptible to CSCtf25886.
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