ProcessBlock: ORPHAN BLOCK 249, prev=1276d27c04c73c7ac48ae11520baf37bdb86c7d3b51b1234d863a3553e296487
ERROR: CheckProofOfStake() : INFO: read txPrev failed
ERROR: AcceptBlock() : check proof-of-stake failed for block f42f133e0cb60b09f83d26fb6e7136992f2afe3ef828196f6533d6acd1aff69d
ERROR: ProcessBlock() : AcceptBlock FAILED
ProcessBlock: ORPHAN BLOCK 250, prev=34f36a06a2d56ce8430905d103b71d0d21104e18fc4553e3604d4a36e4607ec4
GUI: QCoreApplication::postEvent: Unexpected null receiver
GUI: QCoreApplication::postEvent: Unexpected null receiver
ERROR: CheckProofOfStake() : INFO: read txPrev failed
ERROR: AcceptBlock() : check proof-of-stake failed for block f42f133e0cb60b09f83d26fb6e7136992f2afe3ef828196f6533d6acd1aff69d
ERROR: ProcessBlock() : AcceptBlock FAILED
ERROR: CheckProofOfStake() : INFO: read txPrev failed
ERROR: AcceptBlock() : check proof-of-stake failed for block f42f133e0cb60b09f83d26fb6e7136992f2afe3ef828196f6533d6acd1aff69d
ERROR: ProcessBlock() : AcceptBlock FAILED
So, "previous transaction" could not be read. A sign that the current chain you have might be damaged. You should start with a fresh chain and also put some seed nodes into your 8bit.conf.
The other messages like "postEvent: Unexpected null receiver" have nothing to do with blockchains but with the Qt framework. Is basically saying that some window or gadget isn't there anymore to process the "event", that is the mouse click etc.