Had some work done today on the R53, and about 30 miles into my 31 mile drive home the Mini threw a 1237 MIL code. I freaked because the car has been so good in my 1st year of ownership (84000 miles). Read code, clear, inspect.
Multiple threads suggest vacuum leak or pressure sensor for SC.
Anyhow, a vacuum hose came loose on the JCW intake seen in photo below.
Perhaps inadvertently popped loose while it was in the shop. After another 30 miles it hasn't come back. I'll keep you posted.

 

That is an easy one to overlook when replacing the airfilter.

CEL's can give you that sick feeling!

 

Definitely could be a vacuum leak. Looks like you found the issue, but threads below with some more if you run into the code again.

https://www.northamericanmotoring.co...237-again.html
https://www.northamericanmotoring.co...1237-code.html
https://www.northamericanmotoring.co...and-p1237.html