ps one more comment then I will leave it.
the 1st lime this happened ( I'd just finished reading a book in Moon + ) I thought maybe Moon had marked the file as still in use., but then I checked with a different book that had been downloaded to CC but not started, so never read, and got the same result
( an unknown error is preventing... ). it could be that CC had lost storage access permission for its own metadata database, somehow- unsure where that is kept
it was next day that I realised that an attempt i CC to "connect" to cloud also gave a message about folders begin read only and refused to connect. I could nit pick & say that it should still allow connect for
look but not touch... but OTOH it was good to have confirmation of what the issue was, and was more informative than " an unknown error is preventing... "
NB I was able to delete the problem book(s) using moon reader to do the delete or using Xplore, so it seems only CC has gotten its permissions corrupted
So I will stop there, while it is all working again. the only thing that could be worth testing is whether the permissions corruption can be forced to re-occur in android 6 by going to android settings and moving the CC app from main storage to SD card, and back again while the app is in various states ( like still connected, or with a book being read via CC in Moon+
files GO is a google app
https://support.google.com/filesgo/answer/7568243?hl=en
Clear your junk files
Some apps store temporary files. You can free-up space on your device by clearing these files. App settings aren’t affected.
On your Android device, open Files Go Files Go.
At the bottom-left, tap Clean .
On the 'Junk Files' option, tap Free up.
Next to 'Free up', you’ll see the amount of space that you’ll save.
Choose the apps whose files you’d like to clear.
Tap Clear.