I just tried and don't get the error message but the code Xara WDPv11 generates will no longer work as it relies on SWF, which has been deprecated by all browsers.
As Xara uses MS Windows Explorer 11 in its Preview, this is probably why it is failing you.
Instead, create a small Placeholder and use Web Properties > Placeholder > Placeholder > replace with graphic file; link to your required MP4.
This add the MP4 into your design and the Placholder can be hidden from view.
Next, create a larger box for the MP4's display and add this form of code directly into the Placeholder Body section:
<video class="xar_meplayer" src="index_htm_files/<video title>.mp4" controls width = "100%" height="100%"/>
The cost of a Upgrade to v18.5 is USD45, less than a buck a week, which is cheap for any hobby.
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