I've just installed Cura 2.7 having previously used 2.6.2. On loading a model I get the error 'Unable to Slice' because there is a problem with Retraction Speed and related parameters. I don't know why this would affect slicing which worked fine under 2.6.2 with a Retraction Speed set at 25 mm/s. I've tried changing the Retraction Speed but can't get it to work. Any ideas?