Glad it worked for you, and a new cable is always a good thing to try...
i usually see this issue when a rogue tone is installed and accessed....
i suspect that the issue might be the 500 tones that ship with the 500x.
not because of a huge internal difference... simply because the presets
were created on early firmwares and I'm not aware of any updated presets.
the firmware differences can and has been an issue for some hd500 tones.
anyway... just my theory... as the "hold left+power up" trick essentially resets the global settings
and bypasses any disagreeable information that the last used tone may have presented.
Same thing happened to me two days ago. Mine is only 4 weeks old (tomorrow). After the reset I could not get it to sync with the pc. I replaced the USB cable.....then it worked.
So, I am wondering.........does Line6 have a problem with the 500X or do they have a USB cable problem?
This seems to be a common problem with the 500X.