Wrong pdf display in DTPO in El Capitain

Hi there,
For some days now (since the last update?!) my pdfs in DTPO 2.9.7 displays in a wrong manner in the main window - it’s moved to the right, so that only a small part is visible (see attached screnshot). I still use the latest version of Mac OS X El Capitain.

What can I do?

DTPO pdf display.png

After I restarted the App, everything looks fine. PDFs are displayed at the right place.

Any idea what the problem was?