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The reaction cross-section has been measured for deuteron energies below 25 keV in a deuterized Zr target under improved ultra-high-vacuum conditions and controlled target surface contamination. The increase of reaction enhancement factors towards lower energies is much weaker than that determined before and can result not only from the electron screening effect but also from a suggested 0+ threshold resonance in 4He. The cross-section calculations performed within the T-matrix approximation enable to estimate a coherent resonance contribution and explain the observed energy dependence of the enhancement factors. Additionally, indications for the increase of the screening energies due to impurities at the target surface could be found. [ABSTRACT FROM AUTHOR]