NullpointerException when skin properties file exist but cannot be loaded

Error number 4001 in 4: Error while evaluating velocity template login.vm
com.xpn.xwiki.XWikiException: Error number 4001 in 4: Error while evaluating velocity template login.vm
	at com.xpn.xwiki.XWiki.evaluateTemplate(XWiki.java:1890)
	at com.xpn.xwiki.web.Utils.parseTemplate(Utils.java:180)
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:444)
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:191)
	at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:425)
	at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:228)
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913)
	at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:449)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:115)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.java:127)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.xwiki.container.servlet.filters.internal.SetHTTPHeaderFilter.doFilter(SetHTTPHeaderFilter.java:63)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFilter(SavedRequestRestorerFilter.java:208)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:111)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.xwiki.resource.servlet.RoutingFilter.doFilter(RoutingFilter.java:137)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:522)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:1096)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:760)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1480)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
	at com.xpn.xwiki.internal.skin.EnvironmentSkin.getOutputSyntaxString(EnvironmentSkin.java:156)
	at com.xpn.xwiki.internal.skin.AbstractSkin.getOutputSyntax(AbstractSkin.java:140)
	at org.xwiki.rendering.internal.transformation.XWikiRenderingContext.getTargetSyntax(XWikiRenderingContext.java:131)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.evaluateContent(InternalTemplateManager.java:716)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.render(InternalTemplateManager.java:626)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.renderFromSkin(InternalTemplateManager.java:604)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.renderFromSkin(InternalTemplateManager.java:570)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.render(InternalTemplateManager.java:556)
	at com.xpn.xwiki.internal.template.DefaultTemplateManager.render(DefaultTemplateManager.java:65)
	at com.xpn.xwiki.XWiki.evaluateTemplate(XWiki.java:1885)
	... 48 more