Eli’s hunch was correct, it had to do with the RCS tool I used, which was from Cygwin. I installed the binaries Eli pointed to and made sure those were found earlier in the PATH and not it seems to work.
Also, I was a bit embarrassed when I understood that RCS was not an internal tool in Emacs… I don’t know why, but I got the impression that Emacs was handling the RCS stuff internally, so I never suspected that there would be external tools.
Thanks!
/Mathias