Hi Vincent,
Try @Stevie Holloway's solution first because it's the easiest and most direct path if you've got job plan revisioning turned on (you can check in your Organization settings).
If that doesn't work, you may be running into new (maybe not new, but automatically enforced) MAS object structure security. Navigate to the object structure in question (MXL_JOBPLAN) and open "Configure Object Structure Security." Select "Use Object Structure for Authorization Name" and OK when the dialog pops up. You'll notice the READ, INSERT, SAVE, DELETE sig options are now present for the MXL_JOBPLAN object structure. Click OK and navigate to security groups. Open the MAXADMIN security group, navigate to the Object Structure tab and search for the MXL_JOBPLAN object structure. Grant the listed options for the object structure, save and exit/re-enter Maximo. Try loading again.
Thanks,
Matt