With Owncloud 9.1 I found strange error messages in the log caused by the OC background background job:
{"reqId":"H\/geVIF6vE8S\/ooTQMRI","remoteAddr":"","app":"files","message":"Exception: {\"Exception\":\"BadMethodCallException\",\"Message\":\"Not in batch\",\"Code\":0,\"Trace\":\"#0 \\/PATH-TO-OWNCLOUD-INSTALLATION\\/lib\\/private\\/Files\\/Utils\\/Scanner.php(143): OC\\Files\\Cache\\Propagator->commitBatch()\n#1 \\/PATH-TO-OWNCLOUD-INSTALLATION\\/apps\\/files\\/lib\\/BackgroundJob\\/ScanFiles.php(87): OC\\Files\\Utils\\Scanner->backgroundScan('')\n#2 \\/PATH-TO-OWNCLOUD-INSTALLATION\\/apps\\/files\\/lib\\/BackgroundJob\\/ScanFiles.php(111): OCA\\Files\\BackgroundJob\\ScanFiles->runScanner(Object(OC\\User\\User))\n#3 \\/PATH-TO-OWNCLOUD-INSTALLATION\\/lib\\/private\\/BackgroundJob\\/Job.php(52): OCA\\Files\\BackgroundJob\\ScanFiles->run(NULL)\n#4 \\/PATH-TO-OWNCLOUD-INSTALLATION\\/lib\\/private\\/BackgroundJob\\/TimedJob.php(53): OC\\BackgroundJob\\Job->execute(Object(OC\\BackgroundJob\\JobList), Object(OC\\Log))\n#5 \\/PATH-TO-OWNCLOUD-INSTALLATION\\/cron.php(122): OC\\BackgroundJob\\TimedJob->execute(Object(OC\\BackgroundJob\\JobList), Object(OC\\Log))\n#6