Unlock screen is white with no password field visible

[SanderG]

When I want to unlock my iMac (late 2009), after a screensaver was running, Yosemite shows a white screen with my avatar on it.

I can enter my password, but there is no border around the field.
There also is no background image.

Am I the only one with a password-protected screensaver with this issue? It's a built-in screensaver, nothing special.

 

[SanderG]

If I disconnect my external display (LG TV) the issue does not occur anymore.
Hence related to having 2 displays (even with synced display off)

EDIT: the problem returned with the next unlock!