Is the lock cylinder actually turning all the way to the unlock position when you turn the key? This could be a lubrication issue if it isn't. My SDL would allow me to LOCK, but not UN-Lock (key wouldn't turn past ~ 11 o'clock position).

An overnight soak with some spray grease designed for locks and some working back and forth in the morning restored operation of the lock cylinder.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]