Well it isn't as uncommon as we'd all like it to be. I think that it basically means that an entry in the list of objects that comprise the part file , in this case a solid object, has an error on it. Most of these seem to get caused when part of the file is basically corrupted with a bit of information that didn't get written properly due usually to memory errors somewhere in the model's past history. Usually they are fixable.
With the mode of error I would try using the assembly with the deformed part loaded as entire part. Check whether you can alter the deformation parameters. Then if that works we can work on whether you need to turn off partial loading or simple open the component fully as a manual step to make it go away. I suspect that if the parameter for deformation addresses an expression driving a sketch or something else that isn't fully loaded that might be the problem in a nutshell.
If I were going to make a broad recommendation about deformable parts then it would be that if you can create the deformable geometry without using a sketch then I would. This is simply because the deformable geometry as part of a parametric feature based solid may be more likely to be loaded even with partial loading turned on. If I'm wrong on this by all means let be know, but it is what I have been doing and I've had very few problems.
I have seen these errors several times and have had no cases where I could not find some way to fix the data. The most difficult may contain wave linked geometry, but you have made no indication that this is involved so I won't bore you with that.
If you're really stuck then I would start by making a copy of the file, and seeing whether you can delete objects starting with those you least need from the offending file. Do this in stages and test the results and you'll hopefully be able to reduce the file to a bare minimum such that the error object is no longer present.
Best Regards
Hudson
