I have the same problem. I locked bootloader after successful /e/ OS installation using Easy Installer, because it is required by banking apps. No fastboot, no bootloader, only preloader and SP Flash Tool. But SP Flash Tool returns an error on downloading md1img.img from this ROM - STATUS_SEC_INSUFFICIENT_BUFFER. You can see screenshot of the same situation from other Gigaset thread - GS290 - is there a way to get back to factory Android?
I’ve tried a bunch of versions of SP Flash Tool - without success.
Looks like only official verified firmware will save our devices. Is there any chance that the /e/ OS developers will contact with Gigaset developers for verified images and help us?