The problem is caused by Adblock Plus
I remembered that when I installed adblock plus I configured a localhost proxy for it to work. So I tried disabling it and sure enough, the check for updates works again.
I tried adding motorola.com as an exception domain but that didn't make any difference. Does anyone know which domain is used when checking for updates?
Anyway, I thought it was worthwhile to provide the source of the problem in case others run into it and do a search for the error message.