The error message means, that either B or r_prob is not a cell array. Use the debugger to find out, what type it is instead. Type this in the command window:
dbstop if error
Run your code again, when it stops, check the variables:
class(r_prob)
class(B)
Is B pre-allocate as cell before?
Is this a script or a function? In a script you are using the workspace of the caller. So if B was created as a cell array before in the command window by accident, the code ran. But now with a clean workspace the code fails. This is the reason why scripts should be avoided in productive projects, because they cannot be debugged and tested exhaustively and have severe long range side effects.
