Back Up Light On Warning - help please!

Last weekend I noticed the right hand brake light had failed on my sons 2004 CLK500. I duly replaced the P21W and off he went. A couple of hours later he sent me a WA he'd got this warning on the dash. He also says he's now unable to go from D to R without cutting the engine, restarting it in P and only then can he go into R. Any idea what's going on here? Any and all help much appreciated.

Last weekend I noticed the right hand brake light had failed on my sons 2004 CLK500. I duly replaced the P21W and off he went. A couple of hours later he sent me a WA he'd got this warning on the dash. He also says he's now unable to go from D to R without cutting the engine, restarting it in P and only then can he go
into R. Any idea what's going on here? Any and all help much appreciated.

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