thank you very much for your interesting comments and determination.
There are contradictory statements of case length for C. peribenanderi: 17-18 mm (two sources) against 11-12 mm (all other sources). The case of my caterpillar is exactly 10.8 mm long. If this short case length is still compatible with C. peribenanderi, then I think the two cases shown here http://www.lepiforum.de/cgi-bin/lepiwiki.pl?Coleophora_Therinella (feeding on Cirsium arvense) may also belong to C. peribenanderi (instead of C. therinella).
> C. therinella feeds on the seeds of Fallopia.
Yes, that's what I also found in different sources. But is it really beyond doubt?
> I think that your specimen belongs to 2. generation. In Finland
> C. peribenanderi overwinters as a mature larva, without feeding
> in the spring.
Perhaps it was an exception because of the hot weather in July.
Thanks again and best wishes,