According to @Atamari, this query used to run just fine in early September.

Now, it results in Query status: stopped: This query was stopped reproducibly. He tried it several times in the past days, as is evident from the log. A version that I forked yields the same result.

As far as I could find out, the stopped status was added recently (T289349) but I could not figure out what can cause a query to be stopped or why this particular query is not executed.