From exception.log:
2016-01-27 20:28:40 mw1014 commonswiki exception ERROR: [5053f78f] /rpc/RunJobs.php?wiki=commonswiki&type=AssembleUploadChunks&maxtime=30&maxmem=300M MWException from line 549 of /srv/mediawiki/php-1.27.0-wmf.11/includes/context/RequestContext.php: Sessions can only be imported when none is active. {"exception_id":"5053f78f"} [Exception MWException] (/srv/mediawiki/php-1.27.0-wmf.11/includes/context/RequestContext.php:549) Sessions can only be imported when none is active. #0 /srv/mediawiki/php-1.27.0-wmf.11/includes/jobqueue/jobs/AssembleUploadChunksJob.php(37): RequestContext::importScopedSession(array) #1 /srv/mediawiki/php-1.27.0-wmf.11/includes/jobqueue/JobRunner.php(257): AssembleUploadChunksJob->run() #2 /srv/mediawiki/php-1.27.0-wmf.11/includes/jobqueue/JobRunner.php(176): JobRunner->executeJob(AssembleUploadChunksJob, BufferingStatsdDataFactory, integer) #3 /srv/mediawiki/rpc/RunJobs.php(45): JobRunner->run(array) #4 {main}
Somehow the chunked upload assembly jobs have persistent sessions even though they shouldn't.