As rtspsrc queries extensions whether it should configure a stream, the rtspwms extension might indicate that the rtx stream should not be configured (i.e. it returns FALSE), after which another extension (e.g. real rtsp) might simply return TRUE (e.g. if no configure_stream is implemented).

The net effect is that the stream is configured.  On the one hand, this is not the way the extension knowing about the stuff in question intended it.  On the other hand, an inconvenient side effect is that the UDP ports of the RTX stream might be retried for the video stream (rather than those of the audio stream), leading to unsupported transport failure in setting up video stream (due to MS RTSP specs).  So, (usually) video over UDP (for MS WMS servers) might randomly seem to appear or not depending on whether other extensions are present and in what order they are executed.

There may be some way to remedy the latter side effect, though likely no really clean one.  As for the basic problem; maybe gst_rtsp_ext_list_configure_stream should check whether configure_stream method is implemented, or a method should be able to return "do not know/care" ?