We notice http status 500 error for the DependencyHandler url has & in it when the post back event is called in Partial rendering. We reproduce this issue in DNN 9.1.1 vanilla version and get screen shots for it.
You could try the url in the browser and see the error straight away. For e.g
In our case, we are using urlType="Base64QueryStrings" for file processing provider so the dependency handler url is generated with query string parameter.
Please note that this is only happening in post back event of partial rendering. We think that when the js and css are added in server script manager to render dependencies, it doesn't need to replace & with & because the value is literally kept as string and doesn't need to do html escape in this case. We also attached the files that we made fixing.
Let us know if you need more info.