If you are able to open your database in an older version of Password Depot but you can't open it in version 15, first, please make sure whether your old database perhaps is encrypted with both master password and key file. If so, please also use your key file for decryption in order to open your database in version 15, too.
However, if you are sure to have entered everything correctly but you can't open your database in version 15 nevertheless, we recommend to proceed as follows:
1. Open your database in the older version, this means the version you can still open your database with.
2. Export your database from the older version to an XML file by clicking on Tools -> Export.
3. Start version 15 and create a new empty database. Give it any name you want and make sure to select a strong master password.
4. Now, click in version 15 on Tools -> Import and select the import format Password Depot XML Format.
5. Save your imported database as .pswe file.
WARNING: Please delete your XML file from your storage medium after having imported your database successfully, because your entries aren't encrypted in the XML file.