Hi Oscar,
Coincidentally, yes, I have an idea. :-)
Disclaimer: from your last error message I infer that you are working on a Mac OS X system, and I have very little experience with that OS.
The first error message I experienced myself the other day. To cut a long story short, StanHeaders 2.26.28 hit CRAN, and that version is incompatible with rstan 2.26.22 from the Stan repository, but rstan 2.26.23 is now on CRAN. Once one installs Stanheaders 2.26.28 and rstan 2.26.23 from CRAN one should have a working set-up.
I presume you are in the same situation as I was, where you are using a CRAN (mirror) that has already the newer version of StanHeaders but not the newer version of rstan on it.
Full details of my experience are at Beware of StanHeaders (2.26.28) from CRAN if you run rstan 2.26.22 from the Stan repository.
The information in this topic is really out-dated and should not be relied on anymore.
Cheers,
Berwin