Hi Torben,
The APM UI might be reading from incorrect indicies and therefore not get any data. Can you try setting the following in kibana.yml:
apm_oss.errorIndices: apm-*
apm_oss.spanIndices: apm-*
apm_oss.transactionIndices: apm-*
apm_oss.onboardingIndices: apm-*
Thanks