Been having the same issue here since I upgraded 6.11 -> 6.12.15 - specifically my Apps tab was asking me to reinstall the Community Applications plugin but attempts to do that failed with the error 127 message.
I followed cordswires suggestion - renamed upgradepkg- to upgradepkg, then I could successfully reinstall Community Applications, no reboot required - Apps tab is working again now and things SEEM to be OK.