try updating to 2.7.11 which is the latest version. whatever bug you are encountering may have been fixed already.
Thanks but not only it didn't help and the issue happened again - i had to removed all files and restore 2.6.4 wallet from scratch since it somehow could not restart anymore after the wallet was loaded in 2.7.11.
I'm totally lost about this problem, all versions worked like a charm until 2.7.X
you say that you restored from scratch then I'm assuming you have the seed written down? In that case upgrade to 2.7.11 again and this time restore your wallet from seed using 2.7.11. hopefully that'll fix the problem.
you should know that the wallet file format changed in 2.7.x so that's why you can't use a 2.7.x wallet file in previous versions. you can still restore from seed in all versions though.
if restoring from seed in 2.7.11 still doesn't fix your problem than I suggest creating an issue on github:
https://github.com/spesmilo/electrum