Most of the SharePoint Custom Workflow developers might have faced and would be facing an pain killing error while creating a custom approval task edit form and deploying it
The virtual path ‘/_catalogs/masterpage/default.master’ maps to another application, which is not allowed.
Check whether your workflow is deployed in root site,if so just go IIS open the site and _layouts folder select “Folder” which contains the forms and from the properties remove “Application Name” and let it can be root.Since the workflow has been deployed in root folder and if your folder has been made as application SharePoint tries to take a absolute path and maps with different application. Always its recommended to choose “File System” instead of http while you are creating website for building custom the task forms,or association forms to the workflow which is going to be deployed in root site.
Hope this would solved your issue.