@jakoob I have found the problem. The PTFE tubing within the hotend has a flaw in it. I do not know if it happened before assembly of the hotend or was caused by something else.
First let me suggest that as you write the instructions for disassembling the E-axis assembly, have the users remove the heater cartridge and the thermistor from the heat block (see E3D instructions here ).
Having the wires separate from the hotend makes it much easier to disassemble the E-axis assembly and remove the hotend from the Prusa Extruder body. Also, step 18 of the Prusa E-axis assembly instructions is very difficult in reverse. The heatsink of the E3D V6 hotend snaps into a printed ridge on the Extruder body. When I removed it, a portion of the print broke off from the Extruder body.
The E3D V6 hotend has a collet that retains the PTFE tube within the hotend. Mine is broken, with one of the four tabs snapped off. It was inside the heatsink. The E3D instructions show a blue collet clip that locks the collet in place to keep the PTFE tubing from retracting from the hotend. My printer does not have this collet clip.
The problem that kept the filament from extracting or moving forward was that the PTFE tubing has some sort of flaw in it that is slightly darker than the white PTFE, as seen in the photo. Filament cannot pass this flaw. I have no idea what caused it. The Prusa I3 MK3 printed fine for several print jobs before this happened.
If Prusa uses the E3D collet clip, them mine was missing from the beginning.
I am going to need a replacement collet and PTFE tubing before I can reassemble my printer. I will see if E3D has replacement parts.