If this is still happening after a full browser cache refresh, can you let us know which page view this is happening on?
Additionally, if you can send the diagnostics log (which can be downloaded from the Event Log page) to support@finalbuilder.com, we can look if there is anything ususal which may be causing this.
Furthermore, if you are familiar with the web browser development tools, can you open this (F12) and check the Network tab? The bad request should be marked as red - click on it to get the full Response text and send this to us.
Which page(s) is it happening on? is there anything in the eventlog? Can you look in the browser tools console and see what the errror is. We have not seen this here, so we need more detail to be able to resolve this.