In this post we will discuss how to resolve An unknown error has occured in infopath and workflow
The full error message is "An unknown error has occurred. The form template has been published to the server but it can only be opened in infopath filler".
I am not sure wether it is the correct solution or not, but you can try like below: (solution from stackoverflow)
Install "Desktop Experience" which is a Windows feature.
In install Desktop Experience, go to Server Manager > Manage > Add Roles And Features > Features > User Interfaces and Infrastructure > Desktop Experience.
This feature is required to allow SMB-based viewing of SharePoint files (the 'Open in Explorer' option)
Also you can check some of my previous posts on: