I was losing so much work time with Acrobat Pro DC not working that I undertook a complete reset of my PC this weekend and reloaded all programs & apps. This appears to have solved the problem, but I shouldn't have to do this as it's so time consuming; particularly considering the annual charge I pay to Adobe. Looking at one or two forum entries it's clear that this issue is a common one for Adobe and has been going on for some time, at considerable frustration for users. Please feed this back to Adobe and request a fix be found as I would prefer not to have to go through the last few days again.