I found a word an affective workaround in the comments of the Ubuntu's sane-backends bug #1728012. I documented it as a comment on the bug report.

The problem seems to reside in missing symlinks to the epkowa (SEIKO EPSON) libraries in the /usr/lib/x86_64-linux-gnu/sane (or, more generically, /usr/lib/$(uname -m)-linux-gnu/sane) folder.

I thus did the following:

1. sudo ln -sfr /usr/lib/sane/libsane-epkowa* /usr/lib/x86_64-linux-gnu/sane

2. Removed the usb 0x04b8 0x0137 entry I had added to /etc/sane.d/epkowa.conf

3. Added a 55-epson-libsane.rules file to /etc/udev/rules.d/ with the following text:

   SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", MODE=="0666"
   SUBSYSTEM=="usb_device", MODE=="0666"
   
   ATTRS{manufacturer}=="EPSON", DRIVERS=="usb", SUBSYSTEMS=="usb", ATTRS{idVendor}=="04b8", ATTRS{idProduct}=="*", MODE="0666"
   
   ATTRS{idVendor}=="04b8", ATTRS{idProduct}=="0137", MODE="0666", GROUP="scanner", ENV{libsane_matched}="yes"
   

4. Finally I unplugged / plugged my scanner and tried

   scanimage -L
   

   and got this encouraging answer:

   device `epkowa:interpreter:003:011' is a Epson GT-S50 flatbed scanner
   

5. Then I started iscan (Image Scan!) and it immediately detected my scanner. I then could successfully scan a page. Yipee! :-)

UPDATE: (Ubuntu 20.04) I suddenly got the issue back after upgrading to Ubuntu 20.04. I searched quite a bit because nothing seemingly changed in my config... and all of a sudden I noticed that somehow my /etc/sane.d/epkowa.conf file's permissions had been altered and that the file was only readable for root !

After changing it back to "world" readable (chmod +r /etc/sane.d/epkowa.conf ) everything went back to normal and both iscan and scanimage would again be able to detect the scanner properly without having to be invoked using "sudo".

Add a link & a file

The problem stems from missing symbolic links in the /usr/lib/x86_64-linux-gnu/sane (or, more generically, /usr/lib/$(uname -m)-linux-gnu/sane) to the Epson driver directory.

$ sudo ln -sfr /usr/lib/sane/libsane-epkowa* /usr/lib/x86_64-linux-gnu/sane

Furthermore, a 55-epson-libsane.rules file needs to be added to the /etc/udev/rules.d/ directory with the following contents:

    SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", MODE=="0666"
    SUBSYSTEM=="usb_device", MODE=="0666"

    ATTRS{manufacturer}=="EPSON", DRIVERS=="usb", SUBSYSTEMS=="usb", ATTRS{idVendor}=="04b8", ATTRS{idProduct}=="*", MODE="0666"

    ATTRS{idVendor}=="04b8", ATTRS{idProduct}=="0137", MODE="0666", GROUP="scanner", ENV{libsane_matched}="yes"

After power cycling the scanner, the following command should now properly list the connected scanner:

$ scanimage -L

device `epkowa:interpreter:003:002' is a Epson GT-S50 flatbed scanner

Use xsane or the proprietary Epson iscan to operate the scanner.