The bit-error rate (BER) measurements were carried out to evaluate the operation of Adiabatic Quantum-Flux-Parametron (AQFP) circuits at 4.2K. In this study, we measured the BER of an AQFP buffer chain by varying the input current at various operating frequencies ranging from 700 MHz to 2 GHz (see Figure 1). The circuits were fabricated using a low-Jc Nb Josephson integrated circuit process with a critical current density of Jc = 1 kA/cm^2 (1KP), provided by AIST Qufab, targeting operation at qubit temperatures around 20 mK. The measured BER was found to be in good agreement with the simulation results, showing a value of about  10^-9 at an input current of 4 μA. These results confirm that the AQFP circuit has operational stability consistent with the simulations