I got an error message that said "trunk not closed" (even though it was.) Is this a malfunction of the auto-close button? I re-opened and re-closed the trunk using the open & close buttons which didn't help. Any ideas what causes and how to fix?

 

Try reading this...

https://mbworld.org/forums/sl-class-r230/355467-have-trunk-soft-close-trunk-assist-problems-here-how-fix-diy.html

 

You will need to fix the latch, but in the meantime, you can probably get it to close fully if you reset the PSE pump by momentarily pulling its fuse. It's either 71 or 54, depending on model year. Prior to MY2009, it's 71, MY2009 is 54.

 

When the trunk lid closes and latches a microswitch on the latch signals the rear SAM module the closed status. I presume in your case either that swtich, the wiring, or the SAM module is faulty. I would check the microswitch with a multimeter.

 

That is a possibility, but a much more common problem is that the PSE pump shuts off airflow to the latch because it's leaking. Mine does this about one a month. I've just been too busy to pull the latch out and do the repairs noted in the sticky. One way to tell if it's the pneumatic problem is pull the fuse. If it works, then leaks are the problem.

 

The leak you mention is common; I have not known of the microswitch to ever fail. Yet the original post states that the trunk lid is closed and suggests the message is in error. If this is the case, then there is an electrical problem of the sort mentioned in my prior post, and nothing suggests the existence of a pneumatic leak.
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When mine does this, the trunk appears to be closed. It's actually latched and locked, but it lacks less than a centimeter in being fully pulled-down, so it's easy to "assume" that is is closed. Now, this may be because I have the hydraulic trunk closer, which pretty much slams the lid down. If you close by hand, you might not use enough force to engage the latch as you're expecting it to pull down pneumatically. It will be interesting to hear what the OP finds.