I've actually had this error myself, and it's lot specific. I don't know what exactly causes it since I only had it on one lot, but in my case it was a lot where there was a toddler and I tried to get a nanny to come housesit. The nanny never came and I got that message, so my guess is that it's related to the nanny somehow...
Unless, of course, you didn't try that option, in which case it must be something else.