Thank you for posting this! Had the exact same error when trying to import the backup after transferring it via Quick Share. And once you mentioned that Quick Share may not be reliable, I verified that it's really not. Well, it works often, but not for Threema backups, or bigger files. Transferred the same file again via adb and with that the Threema restore worked!
For reference, this is how the backup file has been stored:
$ ls -lgo
-rw-rw---- 1 718645913 2025-09-07 20:39 threema-backup_1757251696181_1_quick-share.zip
-rw-rw---- 1 718845438 2025-09-07 20:34 threema-backup_1757251696181_1_adb.zip
So, for some reason the file from the Quick Share transfer is missing ~194 KB of data, no wonder why the import fails 
Thanks again for mentioning this, glad I found this post!