Thank you for the fast reply. I followed your advice, downloaded the files and installed them in the following order, always setting "Keep settings" to "off":
1. Downgrade to TRB1_R_00.02.06.1
2. Upgrade to TRB1_R_00.07.00
3. Upgrade to TRB1_R_00.07.01
The WebUI always correctly reported the expected FW version after installing the firmware.
The result is that the TRB140 is still running kernel 3.18.20-msm with FW version TRB1_R_00.07.01.