I got this. Also when trying to delete e-cards or switch languages.
I changed the domain to run php as a module instead of cgi and all is golden. There may be other ways to fix the problem I don’t know about or understand.
If you don’t use theme-switching, or e-card logging, or language-switching, evertything else seems to work as cgi. So it’s definitely possible to run CPG as CGI, with limitations.