I’m pretty much out of ideas.
Could you try renaming your model to something else? Like if it is “fido.stan”, rename it to “spot.stan” and try to compile that.
rstan tries to avoid recompiling stuff it has already compiled. By renaming the file we force rstan to do a fresh compile.
Someone reported something like that here: Cannot resolve the "cannot allocate vector of size … " error - #59 by mauthier