For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
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2016-06-09 12:13:06,841 WARN [JAPR_Occurres_] - Raised exception when searching : XSDValidationErrorEx : XSD Validation error(s):
<xsderrors>
<error>
<typeOfError>ERROR</typeOfError>
<errorNumber>1</errorNumber>
<message>cvc-complex-type.2.4.a: Invalid content was found starting with element 'dc:description.abstract'. One of '{"http://purl.org/dc/elements/1.1/":title, "http://purl.org/dc/elements/1.1/":creator, "http://purl.org/dc/elements/1.1/":subject, "http://purl.org/dc/elements/1.1/":description, "http://purl.org/dc/elements/1.1/":publisher, "http://purl.org/dc/elements/1.1/":contributor, "http://purl.org/dc/elements/1.1/":date, "http://purl.org/dc/elements/1.1/":type, "http://purl.org/dc/elements/1.1/":format, "http://purl.org/dc/elements/1.1/":identifier, "http://purl.org/dc/elements/1.1/":source, "http://purl.org/dc/elements/1.1/":language, "http://purl.org/dc/elements/1.1/":relation, "http://purl.org/dc/elements/1.1/":coverage, "http://purl.org/dc/elements/1.1/":rights}' is expected. (Element: dc:description.abstract with parent element: oai_dc:dc)</message>
<xpath>ListRecords/record[1]/metadata/oai_dc:dc/dc:description.abstract</xpath>
</error>
...
2016-06-13 10:33:26,849 WARN [JAPR] - Raised exception when searching : java.lang.NullPointerException
2016-06-13 10:33:26,849 WARN [JAPR] - java.lang.NullPointerException
at org.fao.oaipmh.responses.ListResponse.build(ListResponse.java:139)
at org.fao.oaipmh.responses.ListResponse.<init>(ListResponse.java:66)
at org.fao.oaipmh.responses.ListIdentifiersResponse.<init>(ListIdentifiersResponse.java:54)
at org.fao.oaipmh.requests.ListIdentifiersRequest.execute(ListIdentifiersRequest.java:70)
at org.fao.geonet.kernel.harvest.harvester.oaipmh.Harvester.search(Harvester.java:206)
at org.fao.geonet.kernel.harvest.harvester.oaipmh.Harvester.harvest(Harvester.java:137)
at org.fao.geonet.kernel.harvest.harvester.oaipmh.OaiPmhHarvester.doHarvest(OaiPmhHarvester.java:154)
at org.fao.geonet.kernel.harvest.harvester.AbstractHarvester$HarvestWithIndexProcessor.process(AbstractHarvester.java:494)
at org.fao.geonet.kernel.harvest.harvester.AbstractHarvester.harvest(AbstractHarvester.java:563)
at org.fao.geonet.kernel.harvest.harvester.HarvesterJob.execute(HarvesterJob.java:31)
at org.quartz.core.JobRunShell.run(JobRunShell.java:213)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:557)
2016-06-13 10:33:26,849 ERROR [JAPR] - Unknown error trying to harvest
2016-06-13 10:33:26,849 ERROR [JAPR] - Raised exception when searching