.jpg pictures not displayed in TYPOlight CMS

Dear colleagues,

for four days now all .jpg pictures are displayed as black rectangles in a web site built with a pre- 3.2.2 TYPOlight CMS (yes I know, there's more recent versions out there). It doesn't matter if they are accessed via the normal web site or the TYPOlight backend (management) system. If downloaded, pictures display as usual, without any problem. This indicates that the files exist and are OK; it may be a transfer or rendering problem.
Pictures in .png or .gif format are being displayed correctly.

I'm running out of ideas - any clues?
Cheers
R�diger