I can reproduce this behavior on Apache, both locally and on a Host Europe virtual server.
I fixed it by changing:
RewriteRule ^kirby/(.) error [R=301,L]
to
RewriteRule ^kirby/(.) /error [R=301,L]
… but that would obviously break installing kirby in a subfolder.
Navigating to the /kirby and /site folders correctly shows the error page but appends file system location to the URL, for example:
$ curl -I example.dev/kirby/ HTTP/1.1 301 Moved Permanently … Location: http://starterkit.dev/Users/username/www/starterkit/error
I can reproduce this behavior on Apache, both locally and on a Host Europe virtual server.
I fixed it by changing: RewriteRule ^kirby/(.) error [R=301,L] to RewriteRule ^kirby/(.) /error [R=301,L] … but that would obviously break installing kirby in a subfolder.