Sorry to bump an old post, but adding singular=1e-8 option worked for me. I'm working with a large multiyear agricultural dataset (30 years, 2 sites, 4 blocks, 4 levels treatment 1, 5 levels treatment 2). Spit plot design. I split the data into 4 subsets to make the analysis more manageable (sites analyzed separately, and then around 2004 there was a break in the data due to a summer fallow). The models were much faster this way, and AR1 and ARH1 covariance structures remained options because there was no year gap. The random effects structure gets a little complicated (https://link.springer.com/article/10.1007/s13593-021-00681-4), but 3 of the 4 datasets ran very well with the same code. The fourth dataset just would not run without the infinite likelihood error on iteration 1. No duplicate records that I could find. Adding singular=1e-8 got it to run, and the results look reasonable. Thanks!
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