eventually, I could fix my SDM3055 with the valuable hints in this thread.
After re-formatting a 8GB-USB Stick on raspy linux and copying the 1.01.01.19 FW from a rescue-pack of the German Seller Batronix , the recovery on the front USB succeeded.
Now I wanted to further upgrade to the latest FW version, but without success.
After deleting all files on the USB stick, I copied the latest SDM3055_V1.01.01.27R2.ADS to the root of the stick and followed the instructions.
But the progress bar of the upgrade procedure finished in a second and shortly displayed "Update Fail"
An attempt to upgrade in a sequential way with the .ADS file versions following 1.01.01.19 chronological failed in the very same way.
How can I fix this ?
Thanks for any suggestion,
Joe