Thanks for your post.
Primarily, there are two common reasons for this error. Firstly, you should check if the CostX Database service is running. Open your Services windows and scroll down to “CostX Database” and check the Status. If it is not running, right click and select “Start”.
If you have done this and still get the unable to connect to the database error, then this may be due to a firewall or antivirus software assuming that CostX is a malware. In this case, you will need to either add the Exactal program folder to the AV exclusions list of disable the AV scan and run CostX installer again to repair CostX services.
This issue can also occur if using an older version of CostX after updating to Windows 10, but as you are on Version 6.6 R3, this wouldn’t be the case.
Hope this Helps!
CostX Forum Admin