[#12551] - NullPointerException when printing report

When printing reports we get occasionaly the following error.
This error appears completely randomly.
If we print the same report again. It's normally working.
Then we suddenly get this error on other reports, that worked before.
And if we print them some minutes later. Everything is working again.
I've no clue, what's causing this error.
OS is Windows. The used font is Arial.

Caused by: java.lang.NullPointerException
at net.sf.jasperreports.engine.fonts.FontUtil.getFontInfo(FontUtil.java:195)
at net.sf.jasperreports.engine.fonts.FontUtil.getFontInfo(FontUtil.java:291)
at net.sf.jasperreports.engine.util.JRStyledTextUtil.loadFamilyFonts(JRStyledTextUtil.java:454)
at net.sf.jasperreports.engine.util.JRStyledTextUtil.getFamilyFonts(JRStyledTextUtil.java:441)
at net.sf.jasperreports.engine.util.JRStyledTextUtil.getFamilyFonts(JRStyledTextUtil.java:432)
at net.sf.jasperreports.engine.util.JRStyledTextUtil.resolveFonts(JRStyledTextUtil.java:190)
at net.sf.jasperreports.engine.util.JRStyledTextUtil.getProcessedStyledText(JRStyledTextUtil.java:164)
at net.sf.jasperreports.engine.export.AwtTextRenderer.initialize(AwtTextRenderer.java:71)
at net.sf.jasperreports.engine.export.draw.TextDrawer.draw(TextDrawer.java:71)
at net.sf.jasperreports.engine.export.draw.PrintDrawVisitor.visit(PrintDrawVisitor.java:174)
at net.sf.jasperreports.engine.export.draw.PrintDrawVisitor.visit(PrintDrawVisitor.java:1)
at net.sf.jasperreports.engine.fill.JRTemplatePrintText.accept(JRTemplatePrintText.java:851)
at net.sf.jasperreports.engine.export.draw.FrameDrawer.draw(FrameDrawer.java:266)
at net.sf.jasperreports.engine.export.draw.FrameDrawer.draw(FrameDrawer.java:214)
at net.sf.jasperreports.engine.export.JRGraphics2DExporter.exportPage(JRGraphics2DExporter.java:350)
at net.sf.jasperreports.engine.export.JRGraphics2DExporter.exportReportToGraphics2D(JRGraphics2DExporter.java:316)
at net.sf.jasperreports.engine.export.JRGraphics2DExporter.exportReport(JRGraphics2DExporter.java:231)