I happen to have a similar problem.
I made a factory reset of my Phone (HTC E8) and installed AnkiDroid.
Anyhow, it doesn't even manage to start properly, shouting that "AnkiDroid directory is inaccessible"
Then, I tried to change the folder, but whatever destination I tap in, I get the message ""the path specified wasn't a valid directory".
Basically, I cannot use the app at all after resetting the phone (I resetted again, once from the option, once from twrp, still the same problem.
What can I do...? Learning backlog gets generated for another day now... :'(