Hello 

When I tried to update forum to new IPS - I have an error 500. I decided to restore my site from last backup, but error isn't disappeared.

I have a logs... http://pastebin.com/mYbCNNr1

but I don't understand how to solve that... Maybe problem with my old database? How to check it for any issues? 

Server info: ubuntu 15, vestaCP

Could be a ton of things. I would check error log. In my experience with 4.2.x upgrades this is usually caused by a plug in that needs disabled. Login to the ACP and try disabling any third party plugins. If you cannot get to ACP you can do it from phpmysql.

Whats your old versionnumber and to which version did you try to update?

Did the Upgrader start or when did this error happen (which step) ?

Restore backup, disable all the hooks/plugins and delete all the themes except the default. As some have said it could be a number of things but mine turned out to be the theme causing issues along with navigation icons as a hook.

Best of luck.

I don't have access to my ACP.

Forum have default theme. 

How to disable plugins from phpmyadmin?

Seems that your database is down, you should check if your database is online, and restart it if offline.

Not sure how you manage server but if in doubt a service restart will not hurt. Depending on OS you run something like:

service mysqld stop

 service mysqld restart

Once dB is back up you can look for the table with widgets and plugins. I will look up exact tables when I get back to my desk. Have you tried going to path/admin/upgrade?

Really could be a ton of things. Check the error log and assure that all your services are running.