Problem opening the game

Try verifying your game files through steam.

Library -> Don't Starve Together -> Settings Wheel -> Properties -> Local Files -> Verify Integrity of Game Files.

Try doing it a couple times in a row.

That's literally the only advice I have, but it tends to work for quite a few issues, and it's at least somewhere to start.

In addition to what HumanBean150 provided, check out our troubleshooting guide for this specific issue. This generally is an error message that pops up when something on the computer is blocking the game from being able to successfully read/write/access your hard drive.

Troubleshooting Guide: https://support.klei.com/hc/en-us/articles/360029882171