Hi, I have had the same problem and solved it taking the following steps:
1. Make sure that your work is saved in the translation memory
2. If the source file is a Microsoft word document, open a new document
3. Open the source document, copy it and paste it in the new document you have previously opened and rename it
4. Add that new document to your current Trados project
5. Run batch tasks, so that your new document become a translatable file
6. Open it, as it is now part of the project you were currently working in
7. At that moment the TM should give you back all the segments you have translated so far
8. Save now target as... It should be alright.
Click on the error message, it should display the segment where the error occurs (sometimes the error occurs in one or two segments above or below the displayed segment). Display all tags and try to correct the missing tags or other format settings.