Rayleigh correction of MERIS error

processing: snap -optical-preprocessing-rayleigh correction

data: MER_FR__1PNUPA20050508_024226_000000982037_00075_16662_1971.N1
snap error:An internal error occurred during the target product initialisations.
Type: IllegalArgumentException
Message: The Product ‘MER_FR__1PNUPA20050508_024226_000000982037_00075_16662_1971.N1.rayleigh’ already contains a tie-point grid with the name ‘dem_alt’.

Probably you don’t have the latest version:
This is fixed since quite some time.
You should get the latest version from the download page and after the installation check for updates (menu: Help / Check for Updates).