There is a full chart of all the codes here on Motoring Alliance.
Link to Library Article.
P1685 - Electronic Throttle Control Monitor Level 2/3 Clutch Torque Max Error
As to what is the cause...I'm not sure, hopefully others will chime in with some things to check.
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I just checked with my local MINI shop where I had a RMW tune done and he said with an ECU tune / cold weather, MINI's will occasionally throw the P1685 code.
He said to clear the code and if it does not happen again not to be concerned.
A friend also replied that he find the same answer when he searched the internet.