The previous reload command didn't finish yet after pressing Reload

Hello all.
I hope we will get some help with this issue, because it is making us mad.

~256 extensions
Mozilla Firefox/Google Chrome - both tested

From time to time (in worst moment), when we do some change and press orange Apply changes button in Freepbx, asterisk become hanged out. It means, that no calls can go in or out, extensions become timing out and gettint No service (Aastra phones) on display.
When I’m trying to reload it from console, I’m getting error:
The previous reload command didn’t finish yet
This could be resolved only with hard killall -s SIGKILL asterisk.

Do you have any clue what it could be?
Thanks in advance.