I saw this question elsewhere and the answer was that it was due to overload, like when US opens. This cannot be the case at 3 pm UK time on a Saturday?
If you randomly experience an issue with data requests returning "Backend error. 400 Bad Request", the first thing to do is to make sure you use the latest version of Eikon Data APIs library for Python v1.0.1. The issue with the Web service returning empty response or "Backend error. 400 Bad Request" has been addressed with the release of Eikon Data APIs library for Python v1.0.1 available on PyPI. This does not mean that with Eikon Data APIs library for Python v1.0.1 one can never receive "Bad request" error or that the service can now provide unlimited data downloads. But one should no longer experience random and frequent empty responses or "Backend error. 400 Bad Request" forcing one to resubmit the same requests in a loop until the data is returned.