Steps to replicate the issue (include links if applicable):
After working for a while on SPARQL queries made from the UI (that utilizes the SPARQL endpoint of Lingua Libre), I noticed that at some point it was no longer possible to execute queries any more for some time, causing a "parsererror" problem visible on the "Results" section of the Sound Library.

What happens?:
Looking at the network trace on Firefox, I can see that the endpoint responds with results serialized in XML.

What should have happened instead?:
The endpoint should have responded in JSON. Because that is not the case, we indeed get a parsererror from JQuery.

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
Problem seems to be intermittent. Looks like it is related to Blazegraph. From what I can see on the trace displayed by Firefox, the "Accept: application/json" is there, and after a while Blazegraph will resume its normal behavior and respond with an appropriately serialized response in JSON.