The feature that allows the user to select a print queue on every print job (only available in the Windows client) pops up the dialog behind the vncviewer, at least on my XP machine. Strangely enough, Alingsås/Steria/Attendo says that they are using this feature, and they haven't reported this problem (but others instead, see Issue 11160). See also bug 3078.
*** Bug 3078 has been marked as a duplicate of this bug. ***