+1 for that 7-31 firmware making my Jr. work with my Pocket. (Class 10 Gigastone MicroSD.) And afterward I was also able to use nitro2k01's sgb enabler.Got my Pocket today and also encountered my EZ-Flash Jr not working using the very last release from the testflight. After some testing the newest working firmware is the one from 7-31 (found here). The newer one (Update_FW5_forSGB.gb) does not work and just causes the "error" message.
This firmware works both with my SanDisk Ultra 16GB (Silver, Class 10, A1) and my older Lexar 4GB (Class 4, taken from a New 3DS XL).