What version of Hangfire are you using? I recently upgraded to v.1.5.2 and had similar issues which required me to added specific nonclustered indexes and make most of the NVARCHAR fileds that I converted to VARCHAR back to NVARCHAR…
To give you some context, I manage the HangFire schema manually in order to publish my database project with a db_owner and run my application as a db_read db_write. With this in mind, make sure to have the ‘prepareHangfireSchemaIfNecessary’ set to false on application start. This is a sqlServerStorageOptions property.
Take a look at the Hangfire source code Install.sql and make sure your schema contains the unique nonclustered indexes that are present in this script file. Also check to see if there are other fields within the Hangfire schema that is VARCHAR and change them one at a time to NVARCHAR. Adding the UNCI and change my fields back to NVARCHAR resolved this issue.
Please let me know if you have any questions.