I would fist start up with trying to see if the issue can be replicated with a updated version of JabberVideo (4.7).
Before that continuing to read is not really worth ;-)
Also the MCU and VCS versions might have an impact so you might want to check that as well
and also how the call flow is (like if it uses interworking or not, is a traversal or local call, ...)
Depending on the media flow a trace on the vcs or where ever the media gues through could be also interesting.
Also what kind of computer is used, operating system, network (loss/jitter/...), ...
You can do a wireshark trace on the client site as well as with a switch with a monitor port upfront the mcu.
There are also debugging capabilites on the Movi/JabberVideo client as well as on the MCU
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