@Max Relic as the OP has said everything in the OM had been explored, and as there's signal on the headphone circuit but not apparently on the Line Out circuit, I suspect only 4 other possibilities in addition to those suggested by the Block Diagram or correction with a factory reset:
* The X8 firmware is out of date (or partially corrupted);
* The cable connecting the X8 Line Out jack to the external device is defective;
* The input or output of the external device connected to the X8 is defective in some way (driver, software, setup, hardware)
* The X8 Line Out circuit is defective.
These further muddy the water:
The OP didn't address my questions about the external device and cabling to it; and the OP used the term "file manager" in describing the problem. That term doesn't appear anywhere in the X8 reference manual.
So I don't think there's anything more I can add to this thread, or any point in further speculation.
The OP has been MIA since post #7; so if the OP identifies and resolves the problem, perhaps the OP will come back and share that so others may benefit going forward.