\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) ={\frac {y \left ( x \right ) }{{{\rm e}^{x}}+1}}=0} \]
Mathematica: cpu = 0.333542 (sec), leaf count = 42 \[ \left \{\left \{y(x)\to c_1 \left (e^{-x}+1\right )+c_2 e^{-x} \left (e^x \log \left (e^x+1\right )+\log \left (e^x+1\right )+1\right )\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 27 \[ \left \{ y \left ( x \right ) = \left ( \left ( \left ( {{\rm e}^{x}}+1 \right ) ^{-1}+\ln \left ( {{\rm e}^{x}}+1 \right ) \right ) {\it \_C1} +{\it \_C2} \right ) \left ( 1+ \left ( {{\rm e}^{x}} \right ) ^{-1} \right ) \right \} \]