Connection to server lost while offline

As title says, I had lost connection to server while playing GW campaign off line.

Is this possible? It happened first time after latest build.

the server crashed on your local machine most likely.

What caused it? Does it happen often?

bugs and hopefully not

There are some options to get logs/replays and you can start the sever before trying to run the game.

Most likely cause is propably a out of memory exception. Server needs a whole bunch of extra RAM and processing power in addition to the client. If you don't have at least recommended specs the game will not enable the offline server by default.
Could you upload your DxDiag.txt?

I have 16 GB 2400 MHz ram, don't think that is the problem. Rest of the system is up to date, latest drivers and all.

As it is hot where I live now, I think that maybe overheating after long session (6+ hours) is the reason for the crash. It happened only once.

People with 16 GiB and SSD often make the mistake to not make a (big enough) swapfile. Regardless of RAM, the game does not like having no swap file.
It's one of the things that is easily spotted in a DxDiag log. Please make one and attach it to your post (upload file is between "post reply" and "more options").

Here is dxdiag. I didn't upload it earlier, was at work.

It's unlikely that it was thermally related. If there was a thermal crash, it'd be different, not to mention that your RAM has almost no chance of overheating in normal conditions.

Has the server crashed after this one time? How many offline games have you played total?

About 20 hours, single player game, GW only since the patch. It happened only once. It was on three planet system, I had three subcommanders against three AI. Two of the AIs were defeated and when I started assulting the last one, who was alone on one planet, connection to server was lost and it sent me to main loby.

Likely, it was just a bug that's hard to reproduce. I'd bet that it won't happen again for a while. Also, if it does occur again, try to write down as much about the last 5 seconds as you can remember/know, then try to reproduce it.

I also had this server.exe crash once, in the program log's it said both server.exe and MSVCP110.dll crashed "MSVCP110.dll located in the PA folder.

You can enable logging via a commandline parameter. If you start the server manually before a GW/Skirmish game the PA.exe will try to instantiate another server but use the alrady running one.
Among other things you can get replays and logging done this way:
http://steamcommunity.com/app/233250/discussions/0/619568192902991690/