URL-encoding:

    >> url: http://test/abc/#def/ghi
    == http://test/abc/#def/ghi

If I do it manually, the "%" gets encoded, though:

    >> replace url "#" "%23"
    == http://test/abc/%2523def/ghi

"@" and ":"

    >> read  http://localhost:8008/@xxx:localhost
    ** Access error: cannot connect: tcp://xxx:80 reason: 0
    ** Where: open open unless sync-op either read
    ** Near: open conn port

"#" as part of the url:

    >> write http://localhost:8008/_matrix/client/api/v1/join/#test2:localhost?access_token=shortened ""
    ** Access error: protocol error: "Server error: HTTP/1.0 403 Forbidden"

Testing the same using curl:

    $ curl -XPOST "http://localhost:8008/_matrix/client/api/v1/join/#test2:localhost?access_token=shortened" -d ""
    {"errcode":"M_FORBIDDEN","error":"Missing access token."}⏎           

My guess is that the url is cut at the "#", because of it special meaning in urls. So let's try to url-encode the "#":

    >> write http://localhost:8008/_matrix/client/api/v1/join/%23test2:localhost?access_token=shortened ""
    ** Access error: protocol error: "Server error: HTTP/1.0 403 Forbidden"

Testing again with curl:

    $ curl -XPOST "http://localhost:8008/_matrix/client/api/v1/join/%23test2:localhost?access_token=shortened" -d ""
    {"room_id":"!mLfKqJqpsJduuNgzDy:localhost"}⏎  

So the problem for Rebol seems to be that "#" is not url-encoded, but if I try to url-encode it, the "%" sign gets encoded, so the url is broken again. (As mentioned in the beginning).