https://userapps.support.sap.com/sap/support/knowledge/en/2462664
2462664 - Transaction RZ70: Error "TP_REG_ACCESS_DENIED (thRc=748)" when opening an RFC connection...
Running transaction RZ70 SLD registration, it fails with a message similar to: 0: SAPXXX01_XXX_00 : Used RFC destination: SLD_UC 0: SAPXXX01_XXX_00 : RFC call...