VAST and "No playable sources found"
I'm having a trouble with VAST ad in JWPlayer 6.4.3359 Ads edition. Previously I've successfully configured it for regular, non-VAST video ads, but the time has come for the final test and it failed ;-)
The problem I have is an error message saying "No playable sources found", which - according to the docs  - means no supported-format media sources was found. XML  provided in the config (_tag_ param) contained only 1 video in FLV format, encoded with H.26*4* codec. Since the docs  say, FLV needs H.26*3*, I've got it transcoded to something different - MP4 @H.264 to be precise, but to no avail. What's interesting, both media files play correctly when given in _file_ param (without this whole VAST playlist thing), instead of _advertisement/tag_.
As to JS console, I get _"No suitable players found and fallback enabled"_ message which is weird, because both _flashplayer_ and _html5player_ paths are OK and ordinary (non-VAST) files play just fine.
*Is my XML  correct? Can you spot any mistake in the config ? What am I missing?* What additional info do I need to give you?
Annotations used in the issue description:
 VAST XML sample from the ad server we use (sensitive data redacted) :
bc.. <VAST xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="vast2.xsd" version="2.0">
<MediaFile delivery="progressive" bitrate="300" width="420" height="300" type="video/mp4">
 Simplified JWPlayer config.
jwplayer.key = 'MY.VALID.LICENCE.KEY';