This error usually occurs on new installation.

-17941 CT - This c-tree Server requires a secure logon that your c-tree client library does not support. Update your c-tree client library. 

What we found out that ctodbc9.dll (c-tree client library referenced above in the error message) in Program Files x86\Dentrix folder was not updated during Dentrix update. We suspect that it usually happens on G6 when G4 was updated to G6 directly, or when an update was done incorrectly. So far we had three cases like this.

Dentrix Developer Support recommends checking if Dentrix update is available and run it. This usually doesn't fix the problem.

The solution is to find a good G6 installation and copy ctodbc9.dll from there. Make sure to preserve an existing file in case you need to roll it back. 

Should you have problem finding good G6 installation attached please find version that was used to fix the issue.