Bitcoin stuck, not syncing
A signature check failed for a valid transaction. This likely indicates a CPU or RAM problem with the system you're running on (even though you don't see regular errors in normal operation, Bitcoin does so many computations during validation that it's likely to see them more).

The result of this is that bitcoind marked the chain this transaction was in as invalid, and rejects new blocks in it.

To rebuild your database, you can start the client with the -reindex flag (if you exit while rebuilding, it will pick up where it was stopped on the next run).

One workaround that helps in some cases is running with -par=1, making validation only run in one thread. This seems to reduce the load enough to make some CPUs not cause errors anymore.