Flaky Assert "java.lang.AssertionError: Can't unlock: Not locked!" - h2o

Question

While trying to build a H2O Random Forest via the Python API, I got a flaky error. (I've saved the .err file which is empty and the .out file in case somebody wants to look at it.)
"java.lang.AssertionError: Can't unlock: Not locked!"
in two out of the three times I tried. One failure showed progress up to 86% and the other 90%. On the third try, I got all the way through. I restarted the H2O server after the first failure.
Running on x86_64 x86_64 x86_64 GNU/Linux, Linux 4.4.0-101-generic (Ubuntu), H2O 3.18.0.4, Python 2.7.12
There are about 33K training set examples for a multi-nominal classifier of about 86 classes and 137 numeric input features. Previously, we had no problem (other than some time out issues) with the system using similar data using much older versions of H2O.
Here's the output to stdout from running my program.
Attempting to start a local H2O server...
Java Version: java version "1.8.0_144"; Java(TM) SE Runtime Environment (build 1.8.0_144-b01); Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)
Starting server from /home/ubuntu/django-env/lib/python2.7/site-packages/h2o/backend/bin/h2o.jar
Ice root: /tmp/tmpViqbs4
JVM stdout: /tmp/tmpViqbs4/h2o_ubuntu_started_from_python.out
JVM stderr: /tmp/tmpViqbs4/h2o_ubuntu_started_from_python.err
Server is running at http://127.0.0.1:54321
Connecting to H2O server at http://127.0.0.1:54321... successful.
-------------------------- ----------------------------------------
H2O cluster uptime: 08 secs
H2O cluster timezone: Etc/UTC
H2O data parsing timezone: UTC
H2O cluster version: 3.18.0.4
H2O cluster version age: 24 days
H2O cluster name: H2O_from_python_ubuntu_x4p9wv
H2O cluster total nodes: 1
H2O cluster free memory: 6.545 Gb
H2O cluster total cores: 8
H2O cluster allowed cores: 8
H2O cluster status: accepting new members, healthy
H2O connection url: http://127.0.0.1:54321
H2O connection proxy:
H2O internal security: False
H2O API Extensions: XGBoost, Algos, AutoML, Core V3, Core V4
Python version: 2.7.12 final
-------------------------- ----------------------------------------
[stuff omitted]
Parse progress: |█████████████████████████████████████████████████████████████████████████████| 100%
Running trainh2o()..
drf Model Build progress: |█████████████████████████████████████████████████████████▉ (failed)| 86%
Traceback (most recent call last):
File "/home/ubuntu/XXX/webapp/XXX/classify_unified/buildpc.py", line 130, in <module>
print("\nTrain Error: {}".format(pc.train()))
File "/home/ubuntu/XXX/webapp/XXX/classify_unified/ProvisionClassifier.py", line 2130, in train
self._trainh2o()
File "/home/ubuntu/XXX/webapp/XXX/classify_unified/ProvisionClassifier.py", line 2090, in _trainh2o
training_frame=self._combo_h2odf)
File "/home/ubuntu/django-env/local/lib/python2.7/site-packages/h2o/estimators/estimator_base.py", line 232, in train
model.poll(verbose_model_scoring_history=verbose)
File "/home/ubuntu/django-env/local/lib/python2.7/site-packages/h2o/job.py", line 77, in poll
"\n{}".format(self.job_key, self.exception, self.job["stacktrace"]))
EnvironmentError: Job with key $03017f00000132d4ffffffff$_8f9d9edcff82420eefea1d6cff0f4396 failed with an exception: java.lang.AssertionError: Can't unlock: Not locked!
stacktrace:
java.lang.AssertionError: Can't unlock: Not locked!
at water.Lockable$Unlock.atomic(Lockable.java:197)
at water.Lockable$Unlock.atomic(Lockable.java:187)
at water.TAtomic.atomic(TAtomic.java:17)
at water.Atomic.compute2(Atomic.java:56)
at water.Atomic.fork(Atomic.java:39)
at water.Atomic.invoke(Atomic.java:31)
at water.Lockable.unlock(Lockable.java:181)
at water.Lockable.unlock(Lockable.java:176)
at hex.tree.SharedTree$Driver.computeImpl(SharedTree.java:358)
at hex.ModelBuilder$Driver.compute2(ModelBuilder.java:206)
at water.H2O$H2OCountedCompleter.compute(H2O.java:1263)
at jsr166y.CountedCompleter.exec(CountedCompleter.java:468)
at jsr166y.ForkJoinTask.doExec(ForkJoinTask.java:263)
at jsr166y.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:974)
at jsr166y.ForkJoinPool.runWorker(ForkJoinPool.java:1477)
at jsr166y.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)
H2O session _sid_9f4c closed.