Today I received the following as a part of my opkg update output:
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_packages
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/packages/Packages.sig
Signature check failed.
Remove wrong Signature file.
no issues on my device at all (Ubiquiti RouterStation Pro 19.07.3, ath79 target):
# opkg update
Downloading http://downloads.openwrt.org/releases/19.07.3/targets/ath79/generic/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_core
Downloading http://downloads.openwrt.org/releases/19.07.3/targets/ath79/generic/packages/Packages.sig
Signature check passed.
Downloading http://downloads.openwrt.org/releases/19.07.3/targets/ath79/generic/kmods/4.14.180-1-b84a5a29b1d5ae1dc33ccf9ba292ca1d/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_kmods
Downloading http://downloads.openwrt.org/releases/19.07.3/targets/ath79/generic/kmods/4.14.180-1-b84a5a29b1d5ae1dc33ccf9ba292ca1d/Packages.sig
Signature check passed.
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/base/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_base
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/base/Packages.sig
Signature check passed.
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/luci/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_luci
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/luci/Packages.sig
Signature check passed.
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_packages
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/packages/Packages.sig
Signature check passed.
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/routing/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_routing
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/routing/Packages.sig
Signature check passed.
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/telephony/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_telephony
Downloading http://downloads.openwrt.org/releases/19.07.3/packages/mips_24kc/telephony/Packages.sig
Signature check passed.
The effect was quite persistent until I rebooted the router. After the reboot I repeated opkg update and everything worked fine. Apologies for the confusion!
Just guessing here (way too little info to judge), but could it be possible that you didn't have enough free RAM for that to succeed (and the reboot 'fixing' that)? Package lists, signatures, etc. are downloaded to /tmp/, which is RAM (tmpfs) backed, if you don't have enough free RAM, you'll get weird errors.
My device has 128MB RAM so I do not think I ran off memory. Not sure what it was really. I tried to re-run opkg update several times and even downloaded the signature file with wget and always had the same results. More curiously, all OpenWrt devices behind the router were not able to update correctly. And when I rebooted the main router all problems instantly disappear.