Uninstalling brms and rstan and then reinstalling them from CRAN was a failure. However, resinstalling them both using remotes::install_github("paul-buerkner/brms") was a success. adapt_delta no longer causes problems. So perhaps the issue had to do with some difference between the current version of rstan on CRAN (2.21.2) and the more current version used in the GitHub install (2.26.3).
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