Thanks man,
As a last attempt I thought that I should search file contents for 2.20-r0.10. This resulted:
root@dm8000:/# find / -type f -exec grep -H '2.20-r0.10' {} \;
did opkg update
Tried to install again:
Seems like it worked. Anyway, thank you very much for your support and for pointing me in the right direction.