Thanks,
the rollback helped.
The Adblock version I installed was exactly what was described here: Adblock package release for turris omnia
I downloaded the latest version from the Lede Project download page for the arm_cortex-a9_vfpv3
https://downloads.lede-project.org/releases/17.01.4/packages/arm_cortex-a9_vfpv3/packages/adblock_2.6.2-1_all.ipk
https://downloads.lede-project.org/releases/17.01.4/packages/arm_cortex-a9_vfpv3/luci/luci-app-adblock_git-17.336.23170-d2dc32a-1_all.ipk
I will try it again.