Most likely you will have to delete your data directory and start over
I'm on the 3rd attempt at fully deleting and rewriting -datadir with bitcoin-qt 0.11.0 after having it working in October on bitcoin-qt 0.9.6
HDD hardware seems to be good for everything else.
It is linux, so no antivirus nor malicious software tool to worry about. The linux ufw firewall is active and was correct for 0.9.6
Please can you post a link to recommend a few known-good addnode ipv4 addresses in Dec 2015?
Either that or list some which presently are working with yours.
I've sometimes got it to clear out a few (presumably?) bad ipv4 addresses by stopping it, deleting or moving to backup peers.dat, and restarting, but that does not always work nor does it always go to the same errors. Today I watched the bitcoin-qt window "synchronising with network" for a couple of hours while the block number did not change from 369208, the number of and size of files in the -datadir did not change, and [help][debug console] listed more than 6 connections "in" with various high port numbers and 8 connections "out" port 8333
Stopping it, rebooting and restarting and it did the same but displaying "No Block Source Available" again.