This error is almost always caused by a network/firewall issue. First, ensure the application pool is started. Second, confirm that port 443 and port 80 are open for incoming and outgoing traffic on your firewall. Consider completing the following steps below until the error clears.

1) Recycle the BBIS application pool
2) Restart IIS

Please contact the appropriate software vendor or IT professional for assistance with this process or issue which is beyond Blackbaud's scope of support.