CircleCI killed on Maven

When running Maven on CircleCI, the tests are suddently killed.
(see
https://circleci.com/gh/EmbeddedMontiArc/MontiMath/28)
Here is the configuration file: https://github.com/EmbeddedMontiArc/MontiMath/blob/master/.circleci/config.yml
The same project builds on TravisCI. (see: github/EmbeddedMontiArc/Documentation/blob/master/RepoStatus.md)
Since we do not get any warnings about too much memory or anything similar, we do not know why the build crashes.
Maybe it is a bug, or the configuration file is wrong. But we looked for this issue and tried several things, but it did not help.

We also use the Maven options suggested in When running Maven on CircleCI, the tests are suddently killed.

We also used the Maven options suggested in:
discuss_circleci_com/t/builds-getting-killed-with-vague-message-received-signal-killed/10214

Has anybody any information or tips, we can try?
Thank you very much.

(Could not add all links correctly due to restriction for new users, sorry for the inconvience).