Magento attributes won't save - buttons disabled

I've suddenly started having an odd problem, not something I've seen in Magento before. I'm trying to add a new attribute called 'Brand', which will contain a list of Manufacturers for the client to select one from when creating a new product. The problem is, as soon as I start to add the attribute values, e.g. Sony, Panasonic, Philips etc., and click the 'Save' or 'Save and continue' button, the two buttons become greyed out. Neither the attribute or its values get saved.

This has only just started to happen - it worked fine last week.

The version of Magento is 1.6.2

Does anyone know of a solution to this?

Thanks!