as far as I know the integration was "deprecated" with NAM 2018 release. Please have a look at the "Deprecated features" section of NAM 2018 release notes:
Hello @Friederike T.
Thank you for pointing me out to the link. I found the below information under the deprecated features.
AppMon integration. We encourage customers to migrate from integration with AppMon & UEM data sources towards new integration within Dynatrace. Integration with AppMon & UEM data sources will be removed in the NAM 2019 release
What I understood that the integration has been deprecated from the AppMon 2018 April and later because currently we have DCRUM instead of NAM.
To be more precise, integration between NAM 2018 and AppMon (any version) is deprecated. It will be removed in NAM 2019.
At DC RUM/NAM side we did not test DC RUM 2017 integration with most recent AppMon 2018 releases; however, https://www.dynatrace.com/support/doc/appmon/administration/system-profile/edit-a-system-profile/system-profile-integration/ indicates that it should work (see the bottom of the document).