First of all: SORRY for this question!
But, since when is: "Support Gameboy, Gameboy Pocket, Gameboy Color, Gameboy Advance/SP, even with backlit mods and rechargeable batteries." officially listed in the Feature List of ezflash.cn ??? - https://www.ezflash.cn/product/ezflash-junior/
And what FW and/or Kernel is the best for backlit modded systems? I'am on FW5 and Kernel 1.04e.
Basically, my EZ Jr. boots up and loads/plays games correctly with this FW/Kernel. But if i press the Soft-reset button, it does not reboot correctly into the menu, instead it gets stuck at the "LOADING"-Message.
Are you using rechargeable NiCd batteries by any chance? If so their voltage is known to be a little lower (1.2v x 2 = 2.4v) than your garden variety alkaline AA's (1.5v x 2 = 3v). Rechargeable battery mods that use LiPo packs (3.2-3.7v) are generally the best for a system that draws more power than OEM.
If it happens with regular alkaline AA's then that should still be seen as an issue though...
But if it happens with NiCd batteries... uhh... Who knew undervolting intended operation with increased draw could cause instabilities in systems?! Shame on Nintendo for making such an inferior system!