After re-installing Acrobat DC and the Brother drivers it still didnt work. I decided to try the ADF using an old version of Omnipage. That software asked me to configure and test the Twain driver which I did with the ADF. After that Acrobat worked again with the ADf under TWAIN. So probably something slammed the registry setting for the scanner. After hours of futzking about, it seems to indicate that Acrobat is incapable of checking the driver's health which is probably as it should be but perhaps there should be a preference setting for scanner set-ups instead of trying to do it automatically.