Hi!
For everyone who has lost his ThreemaID because of damaged phone, here is my solution.
All you need is an old TWRP/Nandroid backup that contains the data partition of your phone (you should have installed Threema before you backed up the data :p ) and a software like 7Zip (on Windows) or tar (on Linux).
- Copy the TWRP/Nandroid files to your computer. On my XPeriaZ5 they are named from data.ext4.win000 to data.ext4.win006. These files are normal ".tar.gz" files. Rename them to data.ext4.win000.tgz … data.ext4.win006.tgz.
- Use 7Zip or tar to browse the archive and navigate to "/data/data/ch.threema.app/files/".
- Inside this directory, you will find a file named "idbackup.txt".
- Extract and open this file. It contains your ThreemaID in plain text format.
- Start Threema on your new (unrooted) phone and tell Threema to restore your ID from text.
- Enter the long code you found in idbackup.txt (you will be asked for your password)
- Feel happy