Had the same problem yesterday.
W: GPG error: http://de.archive.ubuntu.com trusty-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <email address hidden>
An hour later the trusty-updates Release.gpg was updated according to the ftp's file timestamp and everything seemed to work properly again.
Had the same problem yesterday.
W: GPG error: http:// de.archive. ubuntu. com trusty-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <email address hidden>
An hour later the trusty-updates Release.gpg was updated according to the ftp's file timestamp and everything seemed to work properly again.