The message is not because a specific version of Bigfix, it is not an error and in general is not something to worry about.
It point out just that one or more agents are requesting for a site ( external, operator or custom ) no more existing on the server, probably because have been deleted.
If the message is visible on the log file of a relay and it is really desired remove them, the two old tech notes in previous comments just suggest a gather reset of that relay and are equivalent to the following KB:
If the message is visible in the server log file instead, the gather reset procedure to be performed is the following ( do not use the procedure for the relay, would leave the server in a inconsistent status ):
but in this second case, even if the procedure can be safely attempted, it is not said it alone would fix the message, so could be necessary open a case and involve the support team.