Coverage line information is garbage

I have a utility class Strings and a test class TestStrings.
I created a run configuration for TestStrings that collects coverage information for Strings.

If I show the coverage information Idea gets the coverage gutter all wrong:
Many comment lines are marked in red and blank lines are green.

I really don't know how to reproduce this: I event tried to copy both files to a new project, that I intended to post together with a Jira issue ... but it worked fine in that test project.

However in my real project I can run the configuration again and again, but never get correct coverage info.

Any hints?