I have been running into the same issue over and over again every time I do a root-to-root migration. It always has to do with the passwords not matching. It is either the Root or Admin accounts get locked out or a command will not run due to the password mismatch.
During the install of a new grid at version 7.1 or higher you are forced to create passwords that has an upper and lower case letter, a number and a special character. The first issue this creates it that if you put the special character at the end of the password, (which is the most common place), the command to start the root-to-root replication interprets the special character as part of the command string and not part of the password and it fails to start. I does seem to work if the special character is in the middle of the password but this does not solve the second issue of password mismatching.
The issue is that during the restore process on the new grid the passwords do not get updated in all the files it is stored correctly. So you now the grid is in a state where some files have the passwords of the new grid and some file have the passwords of the old grid. The fix for this is, that after you install the new grid and have verified that it is working correctly, using the change-password command set all the passwords on the new grid to be the same as the on old grid before doing the root-to-root replication. The change-passwordcommand allows you to set the passwords without having to meet the complexity requirements set during the initial install and it also avoids the issue caused by the use of a special character.
So you will want to follow this process:
Install the new grid and verify it is working correctly
Run the change-passwordcommand and set the new gird passwords to match the old grid
Do the Root-to-Root migration
Complete the restore process on the new grid and verify it is working correctly
Run change-passwordcommand again and set password to meet the complexity requirement used for the install.