When I export my presentation to PDF the first slide, whose master slide has a white and transparent field turns black and transparent.
Anybody has an idea what is going on?
Do you export as PDF/A-1a? Make sure that the PDF/A-1a option is disabled when exporting to PDF, since PDF/A-1a doesn't allow transparent objects:
BTW: This affects every kind of LIbreOffice / OpenOffice.org file exporing to PDF; Calc / Writer files may contain transparent content, too.
As workaround you could try to use cups-pdf.
When installed (re)start LibreOffice, open your document, press Ctrl+P and select "PDF" as printer.
In "Settings" you can change the resolution if you want to; default are 300 DPI.
Click "Print" and you will find the PDF file in ~/PDF.