Today I downloaded SPM 12 latest version, and when I tried to start it complained:
Error using narginchk (line 10) too many input arguments
This error is caused by the conflict of multiple narginchk functions. To find out where they are, type
>> which -all narginchk C:\Users\Xu Cui\Dropbox\spm12\external\fieldtrip\compat\matlablt2011b\narginchk.m built-in (C:\MATLAB\R2015b\toolbox\matlab\lang\narginchk)
Then I simply renamed the first narginchk.m to a different name (e.g. _narginchk.m), and now it works.