Brms control statement causes sampling failure

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When I include a control=list() statement in brms running in RStudio, the model compiles but fails to sample with the following error
SAMPLING FOR MODEL ‘ded7652ad4617eb0e215a91c68d31580’ NOW (CHAIN 1).
[1] “Error in sampler$call_sampler(args_list[[i]]) : "
[2] " VECTOR_ELT() can only be applied to a ‘list’, not a ‘symbol’”
error occurred during calling the sampler; sampling not done

format_code(like_this, if_applicable)
Here's the code and it runs when I comment out the control statement with "#"
form_prior1 <- get_prior(overall_6hr ~ lAge  + lBMI + (1|Gender),
                            family=lognormal() ) 

form_mod1 <- brm(overall_6hr ~ lAge + lBMI + (1|Gender),
                          prior = form_prior1,
                          control = list(adapt_delta = 0.95),
                          chains=2, iter=6000                        

I just updated to Big Sur 11.5.1. The toolchain is fine since the model compiles and runs without the control statement.

Thanks in advance for any insights.

Another user recently reported the same problem here

I don’t know the resolution yet, but let’s move this discussion over there and hopefully we can figure it out, or somebody who already knows the answer will chime in.

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