[Fatal Error] :1:1: Content is not allowed in prolog.

[jonasstein]

In version

JabRef 4.1-dev--snapshot--2017-10-31--master--1ebdea452
Linux 4.12.12-gentoo+ amd64 
Java 1.8.0_152

I get on stderr:

[Fatal Error] :1:1: Content is not allowed in prolog.

Reproduce:
when simply opening the window by clicking on the menu Options/Preferences

12:20:10.329 [AWT-EventQueue-0] WARN  org.jabref.logic.l10n.Localization - Warning: could not get message translation for "Create_blank_entry_linking_the_PDF" for locale en
12:20:10.329 [AWT-EventQueue-0] WARN  org.jabref.logic.l10n.Localization - Warning: could not get message translation for "Create_entry_based_on_XMP-metadata" for locale en
12:20:10.329 [AWT-EventQueue-0] WARN  org.jabref.logic.l10n.Localization - Warning: could not get message translation for "Create_entry_based_on_content" for locale en
12:20:10.329 [AWT-EventQueue-0] WARN  org.jabref.logic.l10n.Localization - Warning: could not get message translation for "Only_attach_PDF" for locale en
[Fatal Error] :1:1: Content is not allowed in prolog.
12:20:11.300 [SwingWorker-pool-4-thread-7] ERROR org.jabref.logic.citationstyle.CitationStyle - Error while parsing source
org.xml.sax.SAXParseException: Content is not allowed in prolog.
	at com.sun.org.apache.xerces.internal.parsers.DOMParser.parse(DOMParser.java:257) ~[?:1.8.0_152]
	at com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl.parse(DocumentBuilderImpl.java:339) ~[?:1.8.0_152]
	at org.jabref.logic.citationstyle.CitationStyle.createCitationStyleFromSource(CitationStyle.java:66) [JabRef--master--latest.jar:?]
	at org.jabref.logic.citationstyle.CitationStyle.createCitationStyleFromFile(CitationStyle.java:98) [JabRef--master--latest.jar:?]
	at org.jabref.logic.citationstyle.CitationStyle.discoverCitationStyles(CitationStyle.java:126) [JabRef--master--latest.jar:?]
	at org.jabref.gui.preftabs.PreviewPrefsTab$1.doInBackground(PreviewPrefsTab.java:201) [JabRef--master--latest.jar:?]
	at org.jabref.gui.preftabs.PreviewPrefsTab$1.doInBackground(PreviewPrefsTab.java:198) [JabRef--master--latest.jar:?]
	at javax.swing.SwingWorker$1.call(SwingWorker.java:295) [?:1.8.0_152]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_152]
	at javax.swing.SwingWorker.run(SwingWorker.java:334) [?:1.8.0_152]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_152]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_152]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_152]
[Fatal Error] :1:1: Content is not allowed in prolog.
12:20:11.310 [SwingWorker-pool-4-thread-7] ERROR org.jabref.logic.citationstyle.CitationStyle - Error while parsing source
org.xml.sax.SAXParseException: Content is not allowed in prolog.