At the beginning bitcoind worked for me without any issuses but now i get the following error at start:

Error: Failed to connect best block

debug.log:
...
ERROR: CheckProofOfWork() : nBits below minimum work
ERROR: CBlock::ReadFromDisk() : errors in block header
ERROR: SetBestBlock() : ReadFromDisk for connect failed
Loading addresses...
...
Done loading
: Failed to connect best block
...
socket send error 9
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Error; StartDumpAddressTimer() failed
...