Nope, that's not the reason either :/
U-Disk is off, and every folder and file on my Dingoo is writeable.
If saving does work once in a while, the emulator will give a message that it's saving the SRAM on quitting. When it doesn't save, there will not be this message.
I am pretty sure it is a bug in the emulators (both Dsnes and Psnes). Saving always works in the original SFC emu as well as any other emulator I installed.