Having this problem on Fedora 19: after leaving a vnc session (set up with tigervnc server and opened with tigervnc’s vncviewer) open for awhile the Gnome 3 automatic lock screen kicks in and refuses to accept a valid password to unlock.
There are more than a few reports on this lodged in RedHat Bugzilla since Fedora 18, but still no resolution (See 912892 as the most on-point). The maintainers of tigervnc tried a patch to their product, but it seems pretty clear that this is actually a Gnome 3 issue and should be addressed by that project.
So far only two workarounds have “worked” for me:
1. Killing and restarting the vnc session as the user from a separate ssh console connection to the target box, and then re-connecting with vncviewer from the remote host.
2. Using loginctl as root on the target box to unlock the session. In theory this require the execution of these commands:
[root@mine ~]# loginctl list-sessions SESSION UID USER SEAT c1 42 gdm seat0 c2 2017 service 27 2000 me 3 sessions listed. [root@mine ~]# loginctl unlock-session 27
Still looking for a better solution here: or at least a non-evasive answer on exactly what the problem is.