Sorry, don’t have time/energy to actually run the model, but some quick thoughts:
That looks like an bug worth reporting at Issues · paul-buerkner/brms · GitHub
Truncation should AFAIK work with neg. binomial models. This looks like Stan is unable to evaluate the gradient of neg_binomial_2_lcdf for your model.This is definitely also a bug (but in the Stan math library) - it’s been discussed before (`neg_binomial_2_lcdf()` "grad_2F1: k (internal counter) exceeded 100000 iterations" - #4 by sakrejda), but I guess it is not yet fixed.
Still I think the fact that you’ve triggered a bug that few people have run into might mean you are exploring some weird parts of the parameter space. Also because you have adapt_delta = 0.99, max_treedepth = 15 I assume you’ve had divergences without that and so there might be something wrong with your model (hard to say what and I encourage you to start a new thread to get more attention on the issue). Possibly if you resolve the original issues, you will no longer hit the bug…
Best of luck with your model!