What relevant tags could be used for this question: android.stackexchange.com/q/120522/96277?

@Firelord Malware? Definitely sounds like that, though OP might not be aware of it.

@Izzy Yes, that could be the case.

@Firelord It obviously was: see OP's feedback (a factory-reset cured it) and my answer.

@Izzy Did and upvoted too, before your comment here. :)

@Izzy Is there a reason flashing boot partition would result in recovery installation? In this question android.stackexchange.com/q/120581/96277 OP flashed boot partition thinking that TWRP would get installed. I can't really get the point.

@Firelord Makes no sense to me either: recovery images IMHO must be flashed to the recovery partition. Guess the guy writing that howto messed up.

@Izzy Thought so.

Bad luck if not even fastboot works. Only solution I could think of would be a bootable SD card – that was how I once recovered a borked tablet (failed update: it wiped everything first, and then figured the images were broken, yuck!)