We calculate the leading order corrections to the [overline B]-->Xsgamma decay in the standard model with two large flat universal extra dimensions. We find that the contributions involving the exchange of Kaluza-Klein modes of the physical scalar field a(kl)[plus-minus] depend logarithmically on the ultraviolet cutoff scale Lambda. We emphasize that all flavor-changing neutral current transitions suffer from this problem. Although the ultraviolet sensitivity weakens the lower bound on the inverse compactification radius 1/R that follows from [overline B]-->Xsgamma, the constraint remains stronger than any other available direct measurement. After performing a careful study of the potential impact of cutoff and higher-order effects, we find 1/R>650 GeV at 95% confidence level if errors are combined in quadrature. Our limit is at variance with the parameter region 1/R<~600 GeV preferred by dark matter constraints.