The error message you get is due to one or more incorrect network settings applied to your Airport Extreme and/or Express with Airport Utility. This may not affect your normal internet use beyond the annoyance of the error popping up. Whatever the case may be, the error can be removed by correcting the proper setting(s) in Airport Utility.
Which particular setting needs fixing depends on exactly how you configured your Extreme and Express with Airport Utility, hence my question about configuration. If you want to get an idea of what could be going on,
Google for the error message, and check out some of the many Apple Discussions threads on the topic.
