Failure from rate-limit

[Peter]

Hi,

 my servers fail to query the upstream servers with these errors:
 
rate-limit: debug 99: rrl=0x0, HAVECOOKIE=0, result=DNS_R_SERVFAIL,
fname=0x8027a5450(0), is_zone=0, RECURSIONOK=1, query.rpz_st=0x0(0),
RRL_CHECKED=0

The operator of the upstream servers says it is due to a configuration
mistake. How can this be fixed? I do not have any rate-limit option
configured...

rgds,
PMc