I've been fiddling around with virtualized OpenWrt 18.06.04 stable quite intensivley for some weeks now.
Since today suddenly there were some luci-app-* packages missing. I firgured
opkg update gave back:
Downloading http://downloads.openwrt.org/releases/18.06.4/packages/x86_64/luci/Packages.gz Updated list of available packages in /var/opkg-lists/openwrt_luci Downloading http://downloads.openwrt.org/releases/18.06.4/packages/x86_64/luci/Packages.sig Signature check failed. Remove wrong Signature file.
Did something go wrong with building packages related to the signature process, recently?