So I was in the nether, building a path, when all of a sudden I couldnt move further. Forcing myself to walk further I take a little bit of suffocation damage (didnt know what the damage came from yet)

Now I took some screenshots. I switched to the default resource pack. Took some more screenshots. I decided to relog. Now a netherrack block I mined had reappeared, right where my head would try to be.

As for the TL;DR description:

I expected: a mined block to be gone, because I mined it.
What happened was: it became invisible, and now allowed me to take suffocation damage, but not to move through it. (around it worked, there was space)
Workaround: relog.

In case this is duplicate: searched for "nether block" "nether invisible"