Hi Lenard,
in cp2k there are two types of optimizations, GEO_OPT and CELL_OPT. In GEO_OPT the lattice vectors are fixed and only the atomic positions are optimized. In a CELL_OPT both the lattice vactors and the atomic positions can be relaxed simultaneously.
So if you don't want the box to change you need
RUN_TYPE GEO_OPT
and the
&MOTION
&GEO_OPT
&END
&END
section.
If the SCF does not converge the forces (and pressure) are probably very bad and thus the optimizer (e.g. BFGS) takes unreasonable steps. If the SCF does not converge you should probably stop the simulation and improve SCF stability.
Cheers,
Fabian