After successful flash no REPL

[freistil]

I have a problem that might have shown up to a couple of people, but I get different errors, so I'll post my case.

After I successfully built the firmware and flashed it onto my board (ESP8266-01 via CH3400 USB-TTL on Debian 9 testing), I tried to access the chip via screen.

Nothing really happened.
After some searching, I realized, the device would appear and disappear regulary for about 1-2 minutes, until it was stable.

I got the idea to catch the output with cat, so this is what I got:
After disconnecting CH_PD, I typed in 'cat /dev/ttyUSB1 115200' and connected the pin to 3.3V, resulting in the following output after some garbage:

"[...]
Ǐ�dc��og'��l�<�l�$����Äl�could not find module 'boot'

>>>
MicroPython v1.6-278-g0cb10b5-dirty on 2016-03-26; ESP module with ESP8266
Type "help()" for more information.
>>> #4 ets_task(401004fc, 3, 3fff9038, 16)
ets_alt_task: ets_run
q for task 3: queue: 3fff9038, get ptr: 0, put ptr: 0, qlen: 16
q for task 20: queue: 3ffefd90, get ptr: 0, put ptr: 0, qlen: 3
q for task 28: queue: 3fff99e8, get ptr: 0, put ptr: 0, qlen: 10
q for cat: 115200: Datei oder Verzeichnis nicht gefunden
"

It seems like the boot module is missing, although I read that this might not be problematic. I'm happy to see that the REPL gets loaded correctly. What do these 'q for task' mean? It looks like the device would power down or reboot, the 'cat 115200: ...' looks like the device has shut down already.
I saw somewhere else the trick to replace some #DEFINE .... 0 with 1, no real help.

What is happening there?

[deshipu]

I think the reason why your USB2TTL is disappearing from the device list is simple -- the ESP8266 requires about 300mA of current, and that's more than your USB2TTL can provide. Either the computer detects overcurrent, and shuts the port off, or the USB2TTL shuts down by itself, to protect your computer.

In either case, try powering the ESP8266 from a separate, stable source of 3.3V. Remember to still connect the ground of the USB2TTL with the ground of the ESP.