If you look at the MANAGE > Nodes view, what is the status of the agent having trouble? Is it showing a 'Green' connected status? Also, is it showing that the Agent version is 6.0.0?
Unless you have excluded the agent from automatic updates, it should have been upgraded to v6.0.0.
Which connector are you trying to add?
Some screenshots would help
It was an issue with the node; C: ran out of space. Thanks!