This is a common problem with these players (first link). My MP860 failed in this way (insufficient voltage etc message)
I made a rig with laboratory PSU (several amp output) connected to USB socket and I know that that the problem has nothing to do with voltage (voltage was stable 5V - tested also higher up to 5.5V)
For a while a trick with holding the volume up button worked for me. (see second link) But once I discharged the battery to a too low level and the trick did not work anymore.
My conclusion is that that message indicates Li-ion battery degradation and charger circuit/FW declines to charge it showing misleading message. (battery made by Foxlink)
I tore my the player open and rigged charge circuit and get it working again.
1. Never allow the battery to be fully discharged. Charge at any opportunity you have.
2. Once you get the Insufficient input voltage message try trick with holding volume adjustment.
3. If 1., 2. failed rip it open and make charging circuit or replace battery.
4. Do not buy Transcend MP3 players. (?)