The macros.conf Stanza [kom_all_assets] should not contain the "| union " SPL command on line 1.
This app is impacted by a SPL xpath command bug (SPL-195545) found in 8.0.x. As a workaround you can switch this app to use python.version = python 2 for the purpose of your testing. Or to be more targeted you can add a command.conf into splunk_kom/local with the following stanza.
filename = xpath.py
streaming = true
retainsevents = true
overrides_timeorder = false
supports_rawargs = true
is_risky = false
python.version = python2
This will simply force the xpath command to run under python 2
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