Reopening, as I have finally managed to reproduce this bug. It occurs for me when the title of a full screen window (the window for which Caffeine inhibits idleness) contains a non-ASCII character. This causes an error in the embedded Perl script in xdg-screensaver, in the subroutine screensaver_gnome_screensaver, where it tries to get the window name (the call to GetProperty). This in turn appears to be a bug in the Perl module X11::Protocol.
Reopening, as I have finally managed to reproduce this bug. It occurs for me when the title of a full screen window (the window for which Caffeine inhibits idleness) contains a non-ASCII character. This causes an error in the embedded Perl script in xdg-screensaver, in the subroutine screensaver_ gnome_screensav er, where it tries to get the window name (the call to GetProperty). This in turn appears to be a bug in the Perl module X11::Protocol.