Typically, this error is caused when the Connector is not installed on the same server as The Raiser's Edge application, they have incompatible versions, or the connection needs to be refreshed between the Connector and the server.

If you are hosted by Blackbaud: Chat With Support and reference this article. 

If you are not hosted by Blackbaud:

Step 1: Verify Versions
  1. In The Raiser's Edge, Select Help and About The Raiser's Edge.  This will need to be 7.91 or higher.
  2. In the Sphere Connector, Select Scheduler.  Then, Select Help > About Sphere Integration to view your version.  This should be the same version as what is listed through  Start>Control Panel>Add or Remove Programs.
    • For The Raiser's Edge 7.91 or higher, the Connector must be 4.0.4.2 or higher
Step 2: Refresh Connection
 
  1. In The Raiser’s Edge, click Sphere, then choose Scheduler.  Select “Configuration”.  Verify the credentials and server name are correct by clicking Finish.  The service host should connect to your Raiser's Edge server and have full access.
  2. In Scheduler, choose File > Stop Host.  Then, choose File > Start Host.
  3. If the stop/start does not work, than go to the server hosting your RE and go then go to Start > Run on the server, then type services.msc.  On this list, double-click “Integration Service [your  Account ID].”  Click Stop. Once it has stopped click Start.
  4. Fully close The Raiser's Edge
  5. Reopen The Raiser's Edge and see if the Scheduler is able to connect to the service successfully. If not, proceed to Step 3.
Step 3: Un-Install and Re-Install the Connector
  1. Please un-install the Connector on the server and install the newest version (service and plug-in).
  2. If that resolves the issue, repeat the process for the plug-in on the appropriate workstations
Step 4: Open and Verify Port and Firewall Settings 
 
  • Destination address
    • 64.209.141.226 is the Blackbaud FTP server
  • Ports to open
    • Outbound: TCP port 21 and TCP ports 1025 and above
    • Inbound: TCP ports 1025 and above
  • For Proxy settings, allow access to 
    • Secftp.kintera.com at 64.209.141.226
    • Connect.kintera.com at 64.209.141.64

ALSO:

Please check that the integration service is using the account ID and not the site ID, this is a common reason for this error.
You can bring up services on the machine running the connector by pressing the windows key and typing in "services" and pressing enter. 
Then you will need to find the "Integration Service(*ACCOUNT*)" service where "*ACCOUNT*" is the account number.
If this number is not the account number, than the connector was installed incorrectly and needs to be reinstalled with the correct account number.