Hello! Sorry to show up again in this manner! But thank you, I am very grateful for the help I put an example of the log on SFS, I know I prefer to look at it directly when helping someone with an error so in case you feel the same, here it is Temps in that one are as follows
MY Want controller 0 = 603
SO Matilda Sim 0 = 10

I got some more info on it today when I was porting in all the kids of the hood, apparently not every child gets it. Out of the 13 I have, about 10 got the error. I'll play around with it some more when I can, see if I can repeat the results on who errors and who doesn't Reading your theories I guess I should mention I use a mod that prevents enthusiasm decay. I don't see why that would result in this as my Sims can and do gain enthusiasm in hobbies, but it's the only thing I can think of that maybe I use that isn't super commonplace

@gummilutt I uploaded some files for you here: https://simfileshare.net/folders/edit/218367/

a) 615_Enthusiasm_GetHobbyIndexinT0.package contains the extracted global BHAV. You could run this file against your mod folder through HCDU. If you find a mod that also overrides this BHAV, it's possible that it indirectly conflicts with my LTW set.
b) ltw_chooser and ltw_support contain the updated "Hobby - Predestined?" BHAV that does not rely too much on the global BHAV 615.
c) LTW_Wellness is updated to rearrange the order of tests. Maybe it'll help if all the aspiration checks run before the hobby check. This will reduce the frequency of error occurrence, if nothing else.

I'm not confident about these solutions, because I can't say I figured out your problem, but we'll see

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UPD. So, we've found out that it was a conflict with Total Anti-Enthusiasm Global mod that indeed overrides BHAV #615. If anyone experiences the same issue as Gummi, go get the ltw_support from the SFS link above. And ltw_chooser, if you use my Lifetime Want Chooser. You don't have to update LTW_Wellness.

Thank you loads Lamare for helping me figure out where this weird error was coming from, and for being so kind as to make alternative versions that avoid the issue caused by Total Anti-enthusiasm mod