  We consider the world surface in AdS_5 that ends on two intersecting null
lines at the boundary. The corresponding string partition function describes
the expectation value of the Wilson line with a null cusp in dual large N
maximally supersymmetric gauge theory and thus determines the cusp anomaly
function f(\lambda) of the gauge coupling. The first two coefficients in its
strong-coupling or string inverse tension expansion were determined in
hep-th/0210115 (a_1=1) and in arXiv:0707.4254 (a_2=- 3 log 2). Here we find
that the 2-loop coefficient is a_2 = - K where K is the Catalan's constant.
This is in agreement (expected on general grounds) with the previous results
for f(\lambda) as the coefficient of log(S) term in the energy of closed
spinning string in AdS_5. The string theory value for a_2 is in agreement with
the numerical result in hep-th/0611135 and the recent analytic result in
arXiv:0708.3933 for the solution of the BES equation following from the
asymptotic Bethe ansatz for the spectrum of the theory. We explicitly verify
the cancellation of 2-loop 2d logarithmic divergences thus demonstrating the
quantum consistency of the AdS_5 x S^5 superstring. We also discuss the
structure of higher loop string corrections to the cusp anomaly giving a 2d QFT
diagrammatic interpretation to the strong-coupling expansion of the cusp
anomaly function as solution of the BES equation found in arXiv:0708.3933.
