[Galaxy S 4 Mini Q&A, Help & Troubleshooting] Mac23:
Inspired by this thread I did some more tests with badblocks on my /data partition.

First pass of badblocks:

~ # badblocks -b 4096 -n -t 0xFF -v -s /dev/block/mmcblk0p24
Checking for bad blocks in non-destructive read-write mode
From block 0 to 1421306
Checking for bad blocks (non-destructive read-write test)
Testing with pattern 0xff: 528100% done, 22:13 elapsed
528108% done, 22:14 elapsed
528112% done, 22:15 elapsed
528116% done, 22:16 elapsed
528120% done, 22:17 elapsed
528124% done, 22:19 elapsed
528128% done, 22:20 elapsed
528132% done, 22:21 elapsed
528136% done, 22:23 elapsed
528144% done, 22:24 elapsed
528152% done, 22:25 elapsed
528180% done, 22:26 elapsed
528188% done, 22:27 elapsed
528192% done, 22:28 elapsed
528196% done, 22:30 elapsed
528200% done, 22:31 elapsed
528208% done, 22:32 elapsed
724709% done, 36:42 elapsed
724710% done, 36:44 elapsed
724711
724716% done, 36:47 elapsed
724717% done, 36:49 elapsed
724718% done, 36:51 elapsed
724719% done, 36:53 elapsed
724720% done, 36:55 elapsed
724721% done, 36:58 elapsed
724722% done, 37:00 elapsed
724723% done, 37:02 elapsed
724724% done, 37:04 elapsed
724725% done, 37:06 elapsed
724726% done, 37:09 elapsed
724727% done, 37:11 elapsed
724728% done, 37:13 elapsed
724729% done, 37:15 elapsed
724730% done, 37:17 elapsed
724731% done, 37:19 elapsed
724732% done, 37:22 elapsed
724733% done, 37:24 elapsed
724734% done, 37:26 elapsed
724735% done, 37:28 elapsed
724736% done, 37:31 elapsed
724737% done, 37:33 elapsed
724738% done, 37:35 elapsed
724739% done, 37:38 elapsed
724740% done, 37:40 elapsed
724741% done, 37:42 elapsed
724742% done, 37:44 elapsed
724743% done, 37:46 elapsed
724744% done, 37:48 elapsed
724745% done, 37:50 elapsed
724746% done, 37:52 elapsed
724747% done, 37:55 elapsed
724752% done, 37:57 elapsed
724753% done, 37:59 elapsed
724754% done, 38:01 elapsed
724755% done, 38:03 elapsed
724760% done, 38:05 elapsed
724761% done, 38:07 elapsed
724762% done, 38:10 elapsed
724763% done, 38:12 elapsed
724788% done, 38:14 elapsed
724789% done, 38:16 elapsed
724790% done, 38:18 elapsed
724791% done, 38:20 elapsed
724796% done, 38:22 elapsed
724797% done, 38:24 elapsed
724798% done, 38:27 elapsed
724799% done, 38:29 elapsed
724800% done, 38:31 elapsed
724801% done, 38:33 elapsed
724802% done, 38:35 elapsed
724803% done, 38:38 elapsed
724804% done, 38:40 elapsed
724805% done, 38:42 elapsed
724806% done, 38:44 elapsed
724807% done, 38:47 elapsed
724808% done, 38:49 elapsed
724809% done, 38:51 elapsed
724810% done, 38:53 elapsed
724811% done, 38:55 elapsed
724816% done, 38:57 elapsed
724817% done, 39:00 elapsed
724818% done, 39:02 elapsed
724819% done, 39:04 elapsed
done
Pass completed, 84 bad blocks found.

Second pass:

~ # badblocks -b 4096 -n -t 0x00 -v -s /dev/block/mmcblk0p24
Checking for bad blocks in non-destructive read-write mode
From block 0 to 1421306
Checking for bad blocks (non-destructive read-write test)
Testing with pattern 0x00: 724716% done, 22:50 elapsed
724720% done, 22:51 elapsed
724724% done, 22:52 elapsed
724728% done, 22:53 elapsed
724732% done, 22:55 elapsed
724736% done, 22:56 elapsed
724740% done, 22:57 elapsed
724744% done, 22:58 elapsed
724752% done, 23:00 elapsed
724760% done, 23:01 elapsed
724788% done, 23:02 elapsed
724796% done, 23:03 elapsed
724800% done, 23:04 elapsed
724804% done, 23:05 elapsed
724808% done, 23:07 elapsed
724816% done, 23:08 elapsed
767717% done, 26:11 elapsed
767718% done, 26:13 elapsed
767719
767724% done, 26:16 elapsed
767725% done, 26:18 elapsed
767726% done, 26:20 elapsed
767727% done, 26:23 elapsed
767728% done, 26:25 elapsed
767729% done, 26:27 elapsed
767730% done, 26:29 elapsed
767731% done, 26:31 elapsed
767732% done, 26:33 elapsed
767733% done, 26:36 elapsed
767734% done, 26:38 elapsed
767735% done, 26:40 elapsed
767736% done, 26:42 elapsed
767737% done, 26:45 elapsed
767738% done, 26:47 elapsed
767739% done, 26:49 elapsed
767740% done, 26:51 elapsed
767741% done, 26:53 elapsed
767742% done, 26:55 elapsed
767743% done, 26:58 elapsed
767744% done, 27:00 elapsed
767745% done, 27:02 elapsed
767746% done, 27:04 elapsed
767747% done, 27:06 elapsed
767748% done, 27:08 elapsed
767749% done, 27:11 elapsed
767750% done, 27:13 elapsed
767751% done, 27:15 elapsed
767752% done, 27:17 elapsed
767753% done, 27:19 elapsed
767754% done, 27:21 elapsed
767755% done, 27:23 elapsed
767760% done, 27:25 elapsed
767761% done, 27:27 elapsed
767762% done, 27:30 elapsed
767763% done, 27:32 elapsed
767768% done, 27:34 elapsed
767769% done, 27:36 elapsed
767770% done, 27:38 elapsed
767771% done, 27:40 elapsed
767796% done, 27:42 elapsed
767797% done, 27:44 elapsed
767798% done, 27:47 elapsed
767799% done, 27:49 elapsed
767804% done, 27:51 elapsed
767805% done, 27:53 elapsed
767806% done, 27:55 elapsed
767807% done, 27:57 elapsed
767808% done, 27:59 elapsed
767809% done, 28:01 elapsed
767810% done, 28:03 elapsed
767811% done, 28:06 elapsed
767812% done, 28:08 elapsed
767813% done, 28:10 elapsed
767814% done, 28:13 elapsed
767815% done, 28:15 elapsed
767816% done, 28:17 elapsed
767817% done, 28:19 elapsed
767818% done, 28:21 elapsed
767819% done, 28:23 elapsed
767824% done, 28:25 elapsed
767825% done, 28:28 elapsed
767826% done, 28:30 elapsed
767827% done, 28:32 elapsed
done
Pass completed, 83 bad blocks found.
~ # fstrim -v /data/
/data/: 9195520 bytes trimmed

Just ignore the output of verbose mode.

It still seems to be a broken eMMC - but the phone is only 2,5 years old. Beside TWRP I never flashed a lot of other things :(