Been having this crash randomly as well since I updated to Crash fixes version 10 (and enabled UseOSAllocators)
I'd been playing for a while without crashes with crash fixes version 8, then I decided to make a new character and updated to crash fixes 10.
I thought I'd maybe hit some sort of cap on the amount of animations Skyrim could handle since I also updated my SLAL packs but even after removing half of those i still kept getting the FootIK crash, but I did notice my game only crashed after registering the SLAL animations to SexLab and restarting my game(so it is probably related to SLAL or SexLab?).
I did not get this crash when I restarted my game without registering the animations in SLAL.
I've now uninstalled Crash Fix all together and just used the Restringer tool to make my game more stable instead. I have not crashed yet.
I've not tested if I get this crash with the UseOSAllocators setting disabled. (would Crash fixes even do anything without that enabled?)
Thank you for making this thread because I'd not thought Crash Fix could possibly be the cause before I saw this and I'd probably still be screwing around trying to find out what I did wrong
Decided to give it another go and installed Crash Fixes again, got rid of a bunch of mods (dual sheath redux among them which is apparently very script heavy). Been playing for most of the day and have yet to get the FootIK crash, so maybe it was just having too many mods.