This error can happen if you move your company file, switch the computer that the Web Connector runs from or if JobNimbus was incorrectly removed from the Web Connector.
First, ensure that you aren't running multiple copies of the Web Connector on different computers. The Web Connector should be installed and running on a single computer. If you're moving the Web Connector to a new computer, first attempt to "Remove" JobNimbus from the Web Connector on the old computer before adding it to the new computer. If something is preventing you from doing this, you can follow the steps in this QuickBooks article to get back up and running:
*Please note that if you do move your QuickBooks company file you should not change the name of the file or you will be required to completely disable the JobNimbus to QuickBooks sync and set it up again from the beginning in order for it to work again.