\[ y''(x)=\frac {y(x)}{e^x+1} \] ✓ Mathematica : cpu = 0.386123 (sec), leaf count = 36
\[\left \{\left \{y(x)\to e^{-x} \left (c_1 \left (e^x+1\right )+c_2 \left (e^x+1\right ) \log \left (e^x+1\right )+c_2\right )\right \}\right \}\]
✓ Maple : cpu = 0.027 (sec), leaf count = 27
\[ \left \{ y \left ( x \right ) ={\frac {{\it \_C1}\, \left ( {{\rm e}^{x}}+1 \right ) \ln \left ( {{\rm e}^{x}}+1 \right ) +{\it \_C2}\,{{\rm e}^{x}}+{\it \_C1}+{\it \_C2}}{{{\rm e}^{x}}}} \right \} \]