@advocatux Thanks again for your support.
Not at the first run, but after some trials I finally got my Aquaris E5 back again!

I'm so happy!

Also helpful were the following links:
https://wiki.ubuntuusers.de/Ubuntu_Touch/flash_tool/
https://forums.ubports.com/topic/1025/ubports-for-bq-e5-all-you-need-to-know/10

For me the issue is fixed and can be closed.