[Paid Software Announcement and Discussion] tobbbie:
Thanks for the hint on the thread for TA restore. Sadly not my option as I do NOT have a TA.img backed up. I was hoping that FlashFire would have the partition included in the "Full" backup I did when I was on LP. Sadly in the meantime I noticed that "Full Backup" for FlashFire does NOT include all partitions which other tools identify - e.g. DiskInfo (pro). The "TA" partion is the first one on the first device (mmcblk0p1). The full list of partitions on (my) Z3 compact is this:

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Internal Storage
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* zram0 [zram0] Not mounted
Total space: 512 MB
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Internal Storage (MMC)
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* TA [mmcblk0p1] Not mounted
Total space: 2 MB
* sbl1 [mmcblk0p2] Not mounted
Total space: 512 KB
* s1sbl [mmcblk0p3] Not mounted
Total space: 256 KB
* dbi [mmcblk0p4] Not mounted
Total space: 64 KB
* aboot [mmcblk0p5] Not mounted
Total space: 512 KB
* rpm [mmcblk0p6] Not mounted
Total space: 512 KB
* tz [mmcblk0p7] Not mounted
Total space: 512 KB
* alt_sbl1 [mmcblk0p8] Not mounted
Total space: 512 KB
* alt_s1sbl [mmcblk0p9] Not mounted
Total space: 256 KB
* alt_dbi [mmcblk0p10] Not mounted
Total space: 64 KB
* alt_aboot [mmcblk0p11] Not mounted
Total space: 512 KB
* alt_rpm [mmcblk0p12] Not mounted
Total space: 512 KB
* alt_tz [mmcblk0p13] Not mounted
Total space: 512 KB
* boot [mmcblk0p14] Not mounted
Total space: 20 MB
* ramdump [mmcblk0p15] Not mounted
Total space: 10 MB
* FOTAKernel [mmcblk0p16] Not mounted
Total space: 16 MB
* DDR [mmcblk0p17] Not mounted
Total space: 32 KB
* LTALabel [mmcblk0p18] (/lta-label) [ext4]
Used: 5,3 MB, Free: 10,7 MB, Total space: 16 MB
* modemst1 [mmcblk0p19] Not mounted
Total space: 1,5 MB
* modemst2 [mmcblk0p20] Not mounted
Total space: 1,5 MB
* fsg [mmcblk0p21] Not mounted
Total space: 1,5 MB
* apps_log [mmcblk0p22] (/idd) [ext4]
Used: 1,6 MB, Free: 6,4 MB, Total space: 8 MB
* System [mmcblk0p23] (/system) [ext4]
Used: 2,2 GB, Free: 325 MB, Total space: 2,5 GB
* Cache [mmcblk0p24] (/cache) [ext4]
Used: 7,4 MB, Free: 192 MB, Total space: 200 MB
* Data (userdata) [mmcblk0p25] (/data) [ext4]
Used: 5 GB, Free: 6,8 GB, Total space: 11,8 GB
* mmcblk0rpmb [mmcblk0rpmb] Not mounted
Total space: 4 MB
--------------------------
SD Card
--------------------------
* SD card [mmcblk1p1] (/mnt/media_rw/30D6-9A71, /mnt/secure/asec, /storage/30D6-9A71) [texfat]
Used: 50,5 GB, Free: 9,1 GB, Total space: 59,6 GB
--------------------------
tmpfs mount points
--------------------------
* /dev [tmpfs]
Used: 140 KB, Free: 871 MB, Total space: 872 MB
* /mnt [tmpfs]
Used: 0 B, Free: 872 MB, Total space: 872 MB
* /tmp [tmpfs]
Used: 0 B, Free: 872 MB, Total space: 872 MB
* /storage [tmpfs]
Used: 0 B, Free: 872 MB, Total space: 872 MB
* /storage/self [tmpfs]
Used: 0 B, Free: 872 MB, Total space: 872 MB
--------------------------
Memory
--------------------------
* RAM
Used: 1,1 GB, Free: 653 MB, Total space: 1,7 GB
* Swap
Used: 212 MB, Free: 299 MB, Total space: 511 MB

On top comes that rooted MM for the Z3C does no more run FlashFire succesfully but restarts the device in the middle before actually backing up anything - logs can be supplied if needed.