This might happen if the XHR process returns a value that is not recognized (such as an echo or var_dump). Without knowing the output of the XHR, there's not much that can be done about this.
This also might occur if your PHP process times out, or if your PHP process runs out of memory. We don't really have much control over these situations. In these cases, you probably won't notice any problems if you have php.ini's display_errors disabled, and even if it's enabled, it's unlikely to be passed to the XHR process in a way that it understands.
And without knowing the process block contents, we don't know what needs to be tweaked to avoid these situations on your installation. When that happens, you'll have to restart the process.