Databases saved as *.pswx files are outdated and have not been recommended for several years now. Thus, the new iOS app version 15 does not support this outdated file format anymore.
If you are using a *.pswx file on your iOS device you will not be able to open it with the latest iOS edition.
However, in order to still have access to your data, please copy your *.pswx file to your computer. Next, open it with the Password Depot desktop program and save it as *.pswe format using the program menu Database -> Save as. Transfer this *.pswe database back to your iOS device and open it with the app.