I have El Capitan 10.11.4 I have the Epson Perfection scanner button problem.
Note, the Epson Perfection V600 is sold on the Apple Store ... so Apple/Epson need to fix this.
PROBLEM DETAILS:
All the Epson scanner drivers and software are up to date on my MacBook Pro mid 2012, but the scanner buttons don't work. When the TWAIN switch alert window pops up, and I press to switch to TWAIN, the Epson Scan button on the scanner works one time. After that, the button stops working. None of the buttons work. I press and nothing occurs at all. The scanner on/off button does work, however.
I restart the computer and scanner and still no go with the buttons. The scanner itself seems to be working fine via ImageCapture or Epson Scan app. It's just the buttons on the scanner.
Again, when I reset TWAIN devices via ImageCapture and then switch back to TWAIN devices as described above in my message here, the scanner button works just one time and then stop working.
I spent about 2 hours with Epson tech support on 4/20/16, but they could not fix the problem. It was quite frustrating. I was using a Perfection V39 at that time. The button works that one time and then nothing. Epson tech support insisted that the scanner is compatible over an over. It clearly is not compatible (at least not yet). I returned the Perfection V39 to Amazon and then purchased the V600 because the V600 is on the Apple Store. I figured it must be compatible. No dice, the V600 has the same scanner-buttons problem as the V39.