Cannot parse joomlaeventmanager.net updatecheck/update_pkg_jem.xml

Check line 37 in \administrator\components\com_jem\models\updatecheck.php file:
This is the place where is defined the url update.Check it in your site, If not, update the line.

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Line 37 in my case reads exactly the same
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The screen copy in a previous reply shows that $updatedata() is NOT filled with the data from the servers xml file

Could I add a statement to updatecheck.php to make it more verbose ?

You are free to play with the code and debug your case.
The first thing is to check the code of the downloaded xml file, is it right? If you have any with the cache provider, please check with github.com/jemproject/JEM-Project/blob/JEM-4.2.2-dev/update_pkg_jem.xml

The reason for the issue as described in reply #4 will remain unknown
Today all the sites in the mentionned domain now detected the update (both Joomla and JEM updatecheck)
and installed it without problems.

Thanks for your efforts