T-Echo no longer responsive

Problem: T-Echo cannot connect via Bluetooth (no BT signal), buttons do not respond, deletion is not possible, new firmware is not possible.

It is possible to go into bootloader mode.
nrf–erase2.uf2 is not running.
There is just the usual error message at the end of the copying process. (PC and Mac)

I tried:
Disconnecting the battery.
Delete and install new firmwarein bootloader mode.

It is firmware firmware-t-echo-2.3.2.63df972.uf2 installed.
The error occurred when I was running MQTT.

Any idea what brings my T-Echo back to life?