OK, I have found the problem.
I use some reporting plugins. In the documentation of the failsafe-maven-plugin I found, that the <encoding> configuration – of course – uses ${project.reporting.outputEncoding} by default.
So I added the property as a child element of the project element and everything is fine now:
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
</properties>
See also How do I prevent “[WARNING] Using platform encoding (CP-1252 actually) to copy filtered resources, i.e. build is platform dependent!”.