Just like the title says, I cannot run or even start the campaign in the Tribes of the East expansion. When I click on the first mission, the game simply crashes. There is no loading screen, no system messages, the game simply closes and that's it. I tried some of the solutions for TotE that other players suggested (like disabling full screen optimization or setting affinity to just one core), but nothing works. I have no problem running the base game, Hammers of Fate, the Legends of the Ancients mod for TotE, or single scenarios in unmodded TotE.

I'm not familiar with that game, but I do know that we're going to need more information, starting with what OS and version you're using. Are you using Window, Mac or Linux, and in each case what version. Also, is your system completely up to date, and if not, what are the most recent updates you've applied? I ask because how we go about troubleshooting this issue depends on what tools are available, and that depends on what system you're using. With luck, somebody will look at your response and know where to go from there.

I'm using Windows 10, and it's currently up to date.

So it turned out that the problem was with the LotA mod after all, or more specifically, with the Campaign.sav file. Since LotA shares the save game directory with unmodded TotE, the game tried to read the Campaign.sav file as it started the TotE campaign, and since that file was from LotA, the game couldn't read it and crashed. When I removed that file from the save game directory, I could finally start the TotE campaign.