grle47, You before tried to install enigma2-plugin-extensions-skin-lib 7.6-r0 version not from feed, but you have not done it correctly.
On my feed is only 7.4-r0 version.
Now opkg trying to install from feed 7.6-r0 version, but can not, because there are not 7.6-r0 version.
Or remove enigma2-plugin-extensions-skin-lib and install 7.4-r0 from the feed, or try correctly install 7.6-r0 version.