It appears that one of the XML files in your dataset is invalid, or had invalid bounding box data. To find out which it is, uncomment this line of code inside the container (i.e. using nano text editor):
Then run train_ssd.py with these options: --batch-size=1 --num-workers=1 --debug-steps=1
The last image info to get printed out before the exception occurs is the one that is causing the problem.
If you continue having issues with it, you can send me your dataset and I can try it.