@VivyanCyril this supports some of my suspicions. We are seeing that the linear mixed effects models is failing for some of the balances. In the OLS command, we have seen this when the variance is close to zero for a balance.
A more sure fire way to debug this sort of problem is to dig into the balance values (as shown in the post here) to see exactly on which value the linear mixed effects model is failing
(here we would use MixedLM instead of OLS.
Forgot to ask in the last question -- did you sanity check this against OLS? If it is also failing on OLS, that would narrow down the potential problems.