A backtrace would be useful, but I think it's hard to create proper backtraces
with Ubuntu packages. You could try to build KOffice from source (./configure
--enable-debug=full) and create a useful backtrace from it. If that's not too
much asked.

I think it is a bit much to ask from a user.  Something that would help almost
as much is to describe the EXACT steps to reproduce the bug.

Reproduce;
1. Fill A1="1", A2="2", A3="=SUM(A1:A2)"
2. Fill A4="=INDIRECT(A3)"

Backtrace;
Using host libthread_db library "/lib/tls/libthread_db.so.1".
[Thread debugging using libthread_db enabled]
[New Thread -1235183936 (LWP 7616)]
[KCrash handler]
#5  0xb61f8618 in KSpread::Value::type (this=0x8352b14) at kspread_value.cc:431
#6  0xb6277b6d in KSpread::ValueConverter::asString (this=0x8121858, 
    value=@0x8352b14) at valueconverter.cc:164
#7  0xb61aa6c6 in func_indirect (args=@0xbfb55b1c, calc=0x8121950, 
    e=0xbfb55c08) at kspread_functions_reference.cc:319
#8  0xb6108d06 in KSpread::Function::exec (this=0x830af98, args=@0xbfb55ef0, 
    calc=0x8121950, extra=0xbfb55c08) at functions.cc:161
#9  0xb6101cd4 in KSpread::Formula::eval (this=0x833c458) at formula.cc:1442
#10 0xb6154a4d in KSpread::Cell::calc (this=0x835f468, delay=true)
    at kspread_cell.cc:2107
#11 0xb6124fe7 in KSpread::Canvas::paintUpdates (this=0x8211f78)
    at kspread_canvas.cc:4115
#12 0xb61fc78c in KSpread::View::paintUpdates (this=0x817b640)
    at kspread_view.cc:7436
#13 0xb6165bd3 in KSpread::Doc::paintUpdates (this=0x8109c60)
    at kspread_doc.cc:1718
#14 0xb6165ce0 in KSpread::Doc::emitEndOperation (this=0x8109c60)
    at kspread_doc.cc:2260
#15 0xb612abeb in KSpread::Canvas::processEnterKey (this=0x8211f78, 
    event=0xbfb56d70) at kspread_canvas.cc:2068
#16 0xb612aeb7 in KSpread::Canvas::keyPressEvent (this=0x8211f78, 
    _ev=0xbfb56d70) at kspread_canvas.cc:2788
#17 0xb6e46756 in QWidget::event () from /usr/lib/libqt-mt.so.3

The crash is fixed now. A lot of thanks to Caleb for the feedback!