ERR_CONNECTION_RESET error after language change

So here is my problem:
I installed latest WP under ENG language. site is up and running. then I logged in to dashboard (successfully) to change the WP language to Russian. after change was applied I still was able to navigate around the dashboard, BUT as soon as I logged out and tried to log in again I get this error “This webpage is not available
ERR_CONNECTION_RESET”

The site is still running just fine with standard theme and no plugins installed.

I searched forums and seems like people do have this problem.
the only way for me to log back in to dashboard was by adding following string my site’s url.
/wp-admin/edit.php?post_type=post
it still asking me to log in but now it doesn’t reproduce the error and gets me straight to POST section, and then I can navigate with no problem.

I changed dashboard back to ENG and now login works just fine. but I need dashboard to be in RU. at least site to be in RU language like Joomla does. ??

So what’s really going on?

Update:I install userPro plugin
and then set it to take over the login process, and now it is okey-ish. I guess.