What's the second non-repeating character?
MATL, 11 bytes
tk&amp;=s1=)FT)

This exits with an error (allowed by default) if there is no second non-repeated character.
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Explanation
t      % Implicitly take input string. Duplicate
k      % Convert to lowercase
&amp;=     % 2D array of equality comparisons
s      % Sum of each column
1=     % True for entries that equal 1
)      % Apply logical index to the input string to keep non-repeated characters
TF)    % Apply logical index to take 2nd element if it exists. Implicitly display