What can I do when the blockchain synchronization is stuck at a specific block? I hadn't started/used my Bitcoin client for ages. Now, whenever I start the client, it is downloading block 135407 and does not progress past it.

What can I do?
I had this happen to me once. It was apparently caused by an incompatible peer node. 


Delete $HOME/.bitcoin/peers.dat
Restart bitcoind


Afterwards, new compatible peers will be found, and hopefully, you'll start processing blocks as expected again.