More specifically, I notice that the block that it has stuck is 13937. Is there any specific issue with this block or it is normal that takes so long?

From the debug.log I get the following:

2017-12-15 21:24:14 WARNING: ProcessBlock(): check proof-of-stake failed for block 312fa8597f54b8453009883f5898c5661565bb0dffa673b03799eb05833af73b
2017-12-15 21:24:14 ERROR: ProcessNewBlock : AcceptBlock FAILED
2017-12-15 21:24:20 ERROR: CheckStakeKernelHash() : min age violation - nTimeBlockFrom=1512045313 nStakeMinAge=3600 nTimeTx=1512048273
2017-12-15 21:24:20 ERROR: CheckProofOfStake() : INFO: check kernel failed on coinstake 340602140fa990574e96896325a9a943879eab740ccdfda1b4483650ffc4bbf3, hashProof=0000000000000000000000000000000000000000000000000000000000000000

2017-12-15 21:24:20 WARNING: ProcessBlock(): check proof-of-stake failed for block 312fa8597f54b8453009883f5898c5661565bb0dffa673b03799eb05833af73b
2017-12-15 21:24:20 ERROR: ProcessNewBlock : AcceptBlock FAILED

Could you please advise?

Thanks in advance!