Getting "programming error: adding reverse spring, setting to unit"

[Raphael Manfredi]

Hello,

I have reorganized a composition of mine.  It was originally
set for a quatuor, but I turned one of the voices into the upper
hand of a piano.

Before that move, everything was right.  Now I get lots of:

programming error: adding reverse spring, setting to unit
continuing, cross fingers

What does this mean??

The result is NOT OK: the space between the clef and the first
note on every staff (but the first) is minimal: it almost touches
the clef.

I use the following skeleton to produce the score (places with "..."
are where I notate the clefs and use the variables for the various
voices:

Music = \new Score <<
  \context StaffGroup = strings <<
        \context Staff = violin { ... }
        \context Staff = viola { ... }
        \context Staff = cello { ... }
  >>
  \context PianoStaff = piano <<
        \context Staff = upper { ... }
        \context Staff = lower { ... }
  >>
>>

\score {
  \Music
  \layyout {}
}

Again, before I added the piano staff part, everything was fine.  But
then I was using the following skeleton:

Music = \new StaffGroup <<
  \context Staff = violinOne { ... }
  \context Staff = violinTwo { ... }
  \context Staff = viola { ... }
  \context Staff = cello { ... }
>>

Is there a workaround?  I'm using version 2.6.3 on Debian (no 2.8.x
out there yet due to dependency problems apparently).

Thanks,
Raphael