Apps Script: We're sorry, a server error occurred

A generic failure inside a Google service call. Google's troubleshooting guide lists the related messages Server not available. and Server error occurred, please try again. and gives three possible causes: a Google server or system is temporarily unavailable, there is an error in your script that does not have a corresponding error message, or there is a bug in Apps Script.

Because the message is generic, the important question is whether it is intermittent. If the same run succeeds a minute later, it was transient. If it fails at the same line every time, something about that call (the data passed, the size of the operation, a deleted resource) is the real problem.

function withRetry(fn, tries = 4) {
  for (let i = 0; i < tries; i++) {
    try { return fn(); }
    catch (e) {
      if (i === tries - 1) throw e;
      Utilities.sleep(1000 * Math.pow(2, i));
    }
  }
}

Expect occasional transient failures in any script that runs unattended. Route service calls through a retry helper with backoff, keep individual operations reasonably sized, and alert yourself on repeated failures rather than single ones.