This is happening when IS is looking at the manifest.v3 file that is in the root of one of the packages. At log level 4 there should be message that states the name of the package being loaded.
One of the manifest.v3 files got corrupted and has a non-terminated XML element called null. Possibly some recent package install or upgrade could have caused the problem or a failed save to an edit on a package property that is stored in the manifest file.
One way to find it is to grep for all manifest.v3 files under the IntegrationServer/packages directory for the string “<null>”.
HTH,
Fred
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