where can i find the crash reports for ark server?

When the ARK server software writes to the log file, it does so by flushing the current log buffer, once it's full. This means that it writes them AFTER the event occured. Due to this, the system itself can't write what caused the crash to the logs, and often times if the log buffer wasn't ufll during the crash, it lacks a LOT of info that occurred prior to the crash. WildCard has somewhat fixed this when the game crashes on your computer, as they use the UE4 launcher wrapper to push out a stack trace of what occurred during the crash. As the server software isn't using a UE4 launcher to start the software (why would it? The server isn't a graphical program after all.) there's not a current work around in place for the server software.

Your best option is to check the option in the Advanced tab to enable server logs, and then you can check the directory called  \ShooterGame\Saved\Logs\ via SFTP. But in the over 5000 hours I've played ARK on this Steam Account I use mainly for admin duties, I have rarely found that the logs yield anything useful.