Brms error: Error in Sys.setenv(R_MAKEVARS_USER = NULL) : wrong length for argument

I tried to use brms for the first time in a while, and I keep getting the same error:

Error in Sys.setenv(R_MAKEVARS_USER = NULL) : wrong length for argument

I have tried it on two different computers (both Windows), both with the most recent R version (4.2.0) and brms version (2.17), and I have tried running it through RStudio and through RGui as well.

The error occurs both with my own code and with the sample code from ?brm:

prior1 <- prior(normal(0,10), class = b) +
  prior(cauchy(0,2), class = sd)
fit1 <- brm(count ~ zAge + zBase * Trt + (1|patient),
            data = epilepsy, family = poisson(), prior = prior1)

Thank you in advance!

Björn