Admins copy the Activate.xml content and save it as a new file, then upload the new file to Server Activation dialogue and there will be “Invalid XML file” error.

We strongly suggest getting the original copy of the XML file, as even a trivial difference between the original file and the new one (even if it is a valid XML format) could cause activation to fail. <xml_time>, <signature>, etc. will get updated every time the file is regenerated.
Admins need to download the Activate.xml file directly and upload it to Strategy Server Activation dialogue for activation, instead of copying the content to a new file to upload.