A similar problem reported in LoadError: PosDefException: matrix is not positive definite; Cholesky factorization failed · Issue #611 · JuliaStats/MixedModels.jl · GitHub turned out to be due to scaling of the response and model matrix - because of weights in that case. That example was a linear mixed model (LMM). With generalized linear mixed models (GLMMs) the problem is even more likely because of the iteratively re-weighted least squares (IRLS) algorithm and its penalized version (PIRLS). Obtaining reasonable starting estimates to avoid this is not easy.