  We have observed the OH 1612-MHz maser emission towards the proto-planetary
nebula candidate OH17.7-2.0 that underwent a very strong and unusual outburst
in 2003. Phase-referencing data were obtained with the EVN in order to localize
the outburst and to examine its possible causes. The majority of the emission
comes from an incomplete spherical shell with inner and outer radii of 220 and
850 mas, respectively. There is a strong evidence for maser components that
arise due to the interaction of a jet-like post-AGB outflow with the remnant
outer AGB shell. The most prominent signature of such an interaction is the
strongly bursting polarized emission near 73.3km/s coming from two unresolved
components of brightness temperature up to 10^11K located at the edge of the
biconal region 2500 AU from the central star. It is remarkable that this OH
biconal region is well-aligned with the polar outflow inferred from the
near-infrared image.
