I had this problem with Nokia 3 where after updating to Oreo my app refused to read and write sd card. I had set the sd to be portable and not internal. I struggled for a month trying every trick to resolve it, I contacted support team severally.
Today I divided to back up my files in of and formated sd as portable storage using the phone. Later I copied back my files from pc. And hurray!!! I now I am enjoying writing file to sd card and I hope the problems I had is resolved