This should be a little workaround-hint. I was now almost 24h withouth a working phone and got the workaround by random. May it helps someone out there.
EDIT: This bug is also on at least another computer with windows and SN-Writer existing.
Symptoms: SP-Flashtools does the flashing very randomly - sometimes, it fails on start, sometimes it comes to the yellow progress-bar and flash some percent, but also fails then. For example, the system-partition never flashed more than 2%. Speed was ~70M/s.
I found stuff like "set sp-flash-tools to Full Speed instead of High Speed - did not work for me.
Solution: For me, this is weird, but as i copied about 3Gb of anything-files to a usual usb-storage (just for the traffic), i retried flashing - and it worked. Speed at ~30M/s. I retried it 3 times, while it was still copy anyhing to the usb-stick, and flashing on the other site seems to get stable - everything is working again.
History behind this: I tried it with two laptops and specialy installed another debian for test. As it did not work, i opened the phone because of the battery, tried with usb-cables etc. In the end, i decided to try Manjaro as another system, with slightly other lib's etc. While copy the image, i had out of random and out of "just another try" (the 1001's), i got it back now.
I really guessed my EMMC-storage is dead, maybe it prevent others also to trash it out of such ideas..
Opinion: Not directly Blackview's fault, but Mediatek one's. Maybe there's a bug-report possible by anyone or i don't know - i have to setup my phone again now