Updating Plugins [Unauthorized]

Upon clicking on Update Plugin I get a message Unauthorized and can not update it.
First that was the case with ‘Error plugin’ which I uninstalled and then reinstalled, but
now same thing happens with Admin Panel to me.

After we added nonces (CSRF form security) in the RC5 release, we missed a few places, and hence these Unauthorized notices. Some more were fixed in RC6 (including the plugin updates). Obviously this is a problem to upgrade to RC6 via the admin, but the workaround is to use GPM that does not use nonces:

$ bin/gpm update