It is a known bug, and frankly, were I JoomlaTools I would not have let customers purchase a product with such a major error. While I'm sure JoomlaTools will fix this, had I known I would not have purchased this component until it had been fixed.
1. Check the support forum first.
2. Wait until a stable release of 1.5 is available
3. Use 1.4 until then
It is a known issue. As mentioned both on our forum and our support portal, the issue will be fixed in 1.5.8. In the meantime, there is a simple patch available: http://support.joomlatools.eu/faq/13-common-errors/155-error-when-clicking-new-doc-in-files-view.html
DOCman Lead Developer