I believe this caterpillar is Achlya flavicornis and probably fallen from a neraby Betula pendula tree.... There are several characteristics which do not match with Polyploca ridens:
e.g. head is not as much nothced as in ridens, ridens often has two light bands on its haed especially visible at the top
the three subdorsal light pinacula are on each segment are roughly the same size whereas in ridens the first piaculum is much smaller
there are black dots above the stigmata, ridens sometimes has black spot below the stigmata
So I wonder if it has been reared but for me it is a doubtful ID.....
All the best,