Can't upload sketch to Tamkyo ESP32-CAM-MB

[jeffas]

I have an ESP32-CAM board connected to a ESP32-CAM-MB, which provides a USB interface and buttons for reset and upload.
It all works fine with the sketch it came with, which provides a WiFi AP and a web server.
I can connect to it using the Arduino IDE (on Xubuntu Linux), and output appears correctly in the serial monitor.
In the IDE, I have identified it as an "AI Thinker ESP32-CAM".
But any attempt to load a new sketch into it fails.
Also, any use of esptool fails.
Here's a typical result from esptool (I tried a variety of commands, and all the various --chip options):

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1. $ esptool.py --chip auto --port /dev/ttyUSB0 read_mac
2. esptool.py v4.7.0
3. Serial port /dev/ttyUSB0
4. Connecting......................................
5.  
6. A fatal error occurred: Failed to connect to Espressif device: Download mode successfully detected, but getting no sync reply: The serial TX path seems to be down.
7. For troubleshooting steps visit: https://docs.espressif.com/projects/esptool/en/latest/troubleshooting.html

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I get the same error message when try to load a sketch from the IDE.
I ran esptool under strace and found that there was definitely data passing both ways through ttyUSB0. I'm not familiar with the protocol, but I can probably capture the traffic if that would be helpful.

Any ideas for possible fixes, or more data to gather?

Update: I read somewhere that one should reset the card after the IDE starts putting out its "Connecting" dots. So I tried that. It worked, but I saw that it had actually started working before the reset. And it's been fine since. So it just suddenly started working without any changes. I guess the moral is: Just keep trying.