As described in the title; the fault itself seems to be related due to a failure with a mixin inject.
I'm not sure if this is something I should report to Sinytra first, but I figured that the details of error would suggest that this is something that needs to be resolved on your end.

I've provided a gist of the error message, as linked below:
https://gist.github.com/GenericCherrySwitch/d9ce3a670659d961c5d5f9b7dd683f3f

@GenericCherrySwitch unfortunately, can't help with that, because Mythic Upgrades uses mixins, which Sinytra can't hold them correctly for now if I remember correctly