software upload to microbit v2 not stable

[rbudde]

1. problem
connect a v2 to the lab using usb. write a simple program. upload it via the file system method (not webusb).

• the program is uploaded and runs
• don't disconnect the v2
• after some time the error 504 is visible on the display
• the file fail.txt on the v2 containd the message 'error: The transfer timed out. type: transient, user'

the upload seems to be not terminated correct. WebUsb is not affected by this error.
reproducable on prod and test.

2. problem
connect a v2 to the lab using usb. Use a current lab version derived from base version 40.
Execute a set to factory default:

• with webusb a flash error is shown in all of my tests.
• with file system upload, a timeout occurs. The default program starts, but terminates later with code 504 in most cases.