if you open Eclipse with an existing project w/o a network connection to the corresponding SAP-system you'll get an errormessage stating that it is not possible to logon. This is just as it should be.
If your networkconnection comes to life ( like establishing a VPN tunnel ) and you retry to logon via Eclipse you'll get the same errormessage even though the connection should be possible. Crosschecking via a "real" SAPGUI shows that the corresponding system is alive and kicking.
Hitting Cancel and retrying to logon will not help. Nor will opening a new window in Eclipse.
The only solution so far is by ending eclipse and restarting.