My node has been stuck on the following error. It was working fine a week ago. What causes this problem and how can this be fixed?

2017-05-20 15:12:23 INFO: Nxt server 1.11.5 started successfully.
2017-05-20 15:12:23 INFO: Copyright © 2013-2016 The Nxt Core Developers.
2017-05-20 15:12:23 INFO: Copyright © 2016-2017 Jelurida IP B.V.
2017-05-20 15:12:23 INFO: Distributed under GPLv2, with ABSOLUTELY NO WARRANTY.
2017-05-20 15:12:23 INFO: Client UI is at https://localhost:6877/index.html
2017-05-20 15:12:23 INFO: RUNNING ON TESTNET - DO NOT USE REAL ACCOUNTS!
2017-05-20 15:12:23 FINE: Known peers: 16
2017-05-20 15:12:26 FINE: Peer{state=CONNECTED, announcedAddress='174.140.168.136', services=29, host='174.140.168.136', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:30 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:35 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:39 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:42 FINE: Peer{state=CONNECTED, announcedAddress='nxt.noip.me', services=31, host='178.150.207.53', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:46 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:48 FINE: Peer{state=CONNECTED, announcedAddress='nxt.noip.me', services=31, host='178.150.207.53', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:53 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:12:55 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:04 FINE: Peer{state=CONNECTED, announcedAddress='52.20.125.165', services=2, host='52.20.125.165', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:09 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:14 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:20 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:22 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:23 FINE: Peer{state=CONNECTED, announcedAddress='174.140.168.136', services=29, host='174.140.168.136', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:28 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:29 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:30 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:34 FINE: Peer{state=CONNECTED, announcedAddress='bug.airdns.org', services=3, host='46.165.208.107', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:36 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:38 FINE: Peer{state=CONNECTED, announcedAddress='nxt.noip.me', services=31, host='178.150.207.53', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
^[^[[D2017-05-20 15:13:39 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246
2017-05-20 15:13:40 FINE: Peer{state=CONNECTED, announcedAddress='107.170.3.62', services=29, host='107.170.3.62', version='1.11.5'} advertised chain with better difficulty, but the last common block is at height 1221246