I Found some unexpected behavior on a DPM server that is configured with an English OS, but with the Dutch regional settings.
When using the command: set-dpmGlobalproperty –DPMserverName <servername> -TapewritePeriodRatio 0.2 (this command configures the DPM settings for marking tapes ready for offsite storage)
This command works and the settings seem to be update.
The workaround is to configure the server with US regional settings at least for the decimal symbol and digit grouping symbol
Thanks to Peer for his help on this.