Sometimes an InDesign document that gets updated frequently or that is especially long can become corrupted. If it's corrupt, it may generate an error when exporting with in5.
If your file that was previously working starts acting up, try saving your InDesign layout as an IDML file. Save that IDML file back as an INDD file, and export it. This process can fix minor corruption in the file and allow you to export without error.
If you're still getting an error when you export, contact us and share your IDML file here: