History Sync fails on master

[SomberNight]

On git master 7ae1a4c

I'm trying to restore a testnet wallet from xpub with ~100 txns spread over several chunks.

Try with a clean datadir.

On some startups of Electrum,

1. sync finishes correctly and it all works as expected.
2. it thinks it finished synchronization (based on GUI status text/icon), but some txns will be left Not Verified in the History tab in QT. Often, the txns in a chunk are either all Not Verified or none of them, but not always :/
3. I get the following trace and sync fails:

Exception in thread Thread-3:
Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/home/user/wspace/electrum/lib/network.py", line 966, in run
    self.wait_on_sockets()
  File "/home/user/wspace/electrum/lib/network.py", line 948, in wait_on_sockets
    self.process_responses(interface)
  File "/home/user/wspace/electrum/lib/network.py", line 612, in process_responses
    self.process_response(interface, response, callbacks)
  File "/home/user/wspace/electrum/lib/network.py", line 558, in process_response
    self.on_get_header(interface, response)
  File "/home/user/wspace/electrum/lib/network.py", line 807, in on_get_header
    chain = blockchain.check_header(header)
  File "/home/user/wspace/electrum/lib/blockchain.py", line 85, in check_header
    if b.check_header(header):
  File "/home/user/wspace/electrum/lib/blockchain.py", line 131, in check_header
    return header_hash == self.get_hash(height)
  File "/home/user/wspace/electrum/lib/blockchain.py", line 266, in get_hash
    assert (height+1) % 2016 == 0
AssertionError

Tested with multiple different servers, the outcomes don't seem to be dependent on them.