A 398 error code indicates that the Dynatrace Performance Network is retrying a request, almost always because of a connection reset but occasionally because of a malformed HTTP reply.
In general, the Agent reports errors when real connection issues occur that would be visible to the end user. Significant browser testing determines many use cases in which a browser gracefully handles connection issues vs. those cases where the end user is presented with the issue.
For example, if a web server claims to support HTTP 1.1 but closes all subsequent connection retry attempts, this triggers a number of issues within a browser. The end user could perceive these issues as connection reset errors.
Internet Explorer features retry the object requests transparently, so the Agent matches this behavior by explicitly reporting the retry as a Synthetic Classic-defined pseudo-redirect with a 398 error code.
Connection failovers that consume time and resources indicate an issue with the back-end infrastructure. The data is presented explicitly so you can investigate the behavior and take action if needed.
However, because the browser does not alert the end user of the retry event, Synthetic Classic does not report this behavior as an error. Synthetic Classic handles the error as a redirect to the same URL, so the software assigns it a specific error code in the 3xx that which describes various redirects.