Dante,
For the design, the only difference with Example 3 is the second factor is also a within-subjects variable.
In your case, you should have 480 input files when there's no missing data if both sstage and cond are within-individual factors. Therefore, the extent of missing data is 1 - 77/480 \approx 84\%. I suspect that this high level of missing data might be the primary reason for the model's failure. OTOH, sstage seems to be a between-individual (instead of within-individual) factor?
As a side note, I recommend considering an improved modeling approach. You might want to explore using 3dLMEr as discussed in this blog post. 3dLME will eventually be phased out.
Gang