As stated above, I compiled the serial library without locks and it works without problems since a week.
Here is the JAR with disabled locks: https://cloud.cc-h.de/index.php/s/u1fbjFhowhlFMjl
It replaces the JAR in this location:
/usr/share/openhab/runtime/system/com/neuronrobotics/nrjavaserial/5.2.1