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IWS has been upgraded to use Microsoft .NET Framework 4.5. This change was made to align with Eagle's core software, in 2015R2 the New UI upgraded to 4.5. If you do not currently have .NET4.5, you will need to download it made in order to access IWS v1.0.1.393 or higher. As of May 13, v1.0.1.393 is now on the IWS Production tools site. You will need to perform a one-time action of uninstalling and reinstalling IWS (tools.eagleaccess.com). You should uninstall using Control Panel > Uninstall a program. To reinstall, you will need to use the new URL for the IWS Tools site: https://tools.eagleaccess.com/iws45/
This issue was tracked as Item #124965.
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