Curation result mismatch between interface/logs and email

[cyplas]

There's a mismatch in (at least some) curation results between interface/logs and email (at least on clarin.si).

For instance, the task "Check that Metadata URLs are working" on handle 11356/1134 yields OK for the handle URL:

2017-09-27 21:53:12,442 INFO  cz.cuni.mff.ufal.curation.LinkChecker @ URL [http://hdl.handle.net/11356/1034] checked in [488292678] nanoseconds
2017-09-27 21:53:12,489 INFO  cz.cuni.mff.ufal.curation.LinkChecker @ URL [https://creativecommons.org/licenses/by-nc-sa/4.0/] checked in [46049415] nanoseconds
2017-09-27 21:53:12,717 INFO  cz.cuni.mff.ufal.curation.LinkChecker @ URL [http://eng.slovenscina.eu/korpusi/proste-zbirke] checked in [228248456] nanoseconds
2017-09-27 21:53:12,717 INFO  org.dspace.curate.Curator @ Curation task: fastchecklinks performed on: 11356/1034 with status: 1. Result: 'Item: 11356/1034  [https://www.clarin.\
si/repository/xmlui/admin/item?itemID=1554] has 3 urls to check...
 - http://hdl.handle.net/11356/1034 = 200 - OK
 - https://creativecommons.org/licenses/by-nc-sa/4.0/ = 403 - FAILED
 - http://eng.slovenscina.eu/korpusi/proste-zbirke = 200 - OK

But I still get an email:

Error: url="https://www.clarin.si/repository/xmlui/handle/11356/1034", query_string="null", ip="95.87.154.205", host="95.87.154.205", user-agent=""DSpace Link Validator""
java.util.EmptyStackException
        at java.util.Stack.peek(Stack.java:102)
        at java.util.Stack.pop(Stack.java:84)
        at org.apache.cocoon.callstack.CallStack.leave(CallStack.java:54)
        ...

This happens consistently for this handle, but it certainly doesn't happen for every handle. For some items, the results are consistent, with both methods failing (e.g., 11356/1109) or both succeeding (e.g., 11356/1033). And all these items have the license 403 problem (#678), so it can't simply be due to that.

In our local config, we have (I think these are the defaults anyway):

lr.link.checker.user.agent = "DSpace Link Validator"
lr.link.checker.connect.timeout = 2000
lr.link.checker.read.timeout = 3000