Symptoms
Emails do not send, the error in the HostServer log is Value cannot be null.
Parameter name: connectionString
Diagnoses
This is caused by a blank connection string
Resolution
Open the Connectionstringeditor.exe located in [INSTALLDIR]K2 blackpearlHost Serverin and click on the connection strings until you find the one that is missing details.
Either fill in the details as appropriate or delete the connection string. Save, and restart the K2 service.