Catscan2 and QuickIntersection are powerful tools to interrogate the notoriously difficult category tree system on Wikipedia and other Wikimedia projects. In order to do that, these tools need a lot of compute resources. They are also very popular.
The webservice that Wikimedia Labs uses to run tools is, apparently, not up to the task in this, and a few other, cases. The webservice crashes, or runs out of resources; Labs root would know. Also, the webservice does not restart automatically after crashing. I, and others, have pleaded with the WMF to fix that, to no avail. A Labs service called "Big Brother" (of all things...) is supposed to help with that, but does nothing for this tool.
I keep getting notices on Twitter, mail, talk pages, bugzilla, and who-knows-what whenever this or other tools are down. Which, in principle, is fine, however:
Dear Wikimedia Foundation: I have a job. Occasionally, I have a live. I do not have time to manually restart the tools I wrote every few minutes because you can't be bothered to provide a webservice that works in real life.
Thus, this tool will remain offline until the WMF fixes the issue. The code is here, in case anyone wants to run it elsewhere. Apologies for the inconvenience. Best of luck to you.
I'll leave QuickIntersection running for now. Sorry, not even that works with this webservice.