  A measurement is presented of dijet and 3-jet cross sections in low-|t|
diffractive deep-inelastic scattering interactions of the type ep -> eXY, where
the system X is separated by a large rapidity gap from a low-mass baryonic
system Y. Data taken with the H1 detector at HERA, corresponding to an
integrated luminosity of 18.0 pb^(-1), are used to measure hadron level single
and double differential cross sections for 4<Q^2<80 GeV^2, x_pom<0.05 and
p_(T,jet)>4 GeV. The energy flow not attributed to jets is also investigated.
The measurements are consistent with a factorising diffractive exchange with
trajectory intercept close to 1.2 and tightly constrain the dominating
diffractive gluon distribution. Viewed in terms of the diffractive scattering
of partonic fluctuations of the photon, the data require the dominance of
qqbarg over qqbar states. Soft colour neutralisation models in their present
form cannot simultaneously reproduce the shapes and the normalisations of the
differential cross sections. Models based on 2-gluon exchange are able to
reproduce the shapes of the cross sections at low x_pom values.
