I experienced the same error working with the Object Loader and instead I used the Append tool in ArcCatalog, it worked well importing the annotation layer into an SDE db dataset.
Good luck!
Hello Sendhil,
There is a chance you are running into an older 10.0-10.2 defect:
The workaround for this defect was to attempt the load operation through ArcCatalog instead of ArcMap. With that in mind- I'd consider trying the load through Catalog to see if that works for you. If it does, it's likely related to the above bug- if not, then there might be something annotation or data specific prohibiting the load from completing successfully. I hope this is helpful!
Not sure if this is the reason you were running into the issue on your end, but I was having the same error and it turned out to be because the feature class I was trying to load data into wasn't versioned.
Hope this helps someone.