Hi found the solution to the pesky lock symbol on the net, thought I share it with you so the solution spreads :
Press and hold the two bottom buttons (4+5) for 15 seconds. the lock symbol will "unlock" after exactly 15 seconds and you are now able to use the OSD menue again!
Funky part is that this is not mentioned in the Dell U2711 manuals (haven't been able to find anything about it). Found the solution via a google search