The error affects use of Mockito on Java 15 and can be reproduced reliably. It seems like it happens if an unnamed module is produced for the first time for a class loader by using reflection.

To create the error, with a recent Java 15 as JAVA_HOME, execute:

git clone https://github.com/scm-manager/scm-manager.git
cd scm-manager
./mvnw clean install -pl :scm-webapp