Hi Timo,
You need to set in each module a relative path for sonar.coverage.jacoco.xmlReportPaths
to the aggregated report using ../
e.g.:
<sonar.coverage.jacoco.xmlReportPaths>${project.basedir}/../path_to_module_with_report/target/site/jacoco-aggregate/jacoco.xml</sonar.coverage.jacoco.xmlReportPaths>`
See § Multi-module builds
of [Coverage & Test Data] Importing JaCoCo coverage report in XML format