While attempting to install Onyx 4.2.1052 on Windows 10 Build 1809 I receieved the common 1720 and 27502 errors. After emailing the support and looking around the forms the only help I found was the V1 install cleaner and the the Dolby Reg fix (Not sure if this actually did anything but it doesn’t hurt).
Once I did the cleaner and Reg fix my 1720 was gone and left me with “Error 27505. Could not connect to Microsoft SQL Server ‘(localdb). \ObsidianOnyx’, Login timeout expired (0)”
What I did to fix that was to open PowerShell(or CMD) as Admin, run ‘sqllocaldb info’. If anything was listed regarding Onyx like “.\ObsidianOnyx” or “ObsidianOnyx” then delete it by running “sqllocaldb d .\ObsidianOnyx” Next run the V1 Cleaner and the start the Onyx Installer. Hopefully that fixes any issues.