Naomi,
Save a copies of your Transformation and Conversion files, and then let the system make the changes to your files. Then, you can compare the original files to the changed ones to maybe get a better idea of what is going on. Before doing that, you should be able to see something like the first 200 rows of errors from the validation screen, which may be enough for you to determine the issue.
You do not have anything mapped for the account dimension, which looks like that could be the issue. You will need to have every dimension in the target model noted in the mapping section of the transformation file.
Jeff