First, I am assuming that the device is not rooted, you upgraded from Kitkat, and are using CC's private folder on the SD card. If one of these is not true then the problem comes from another area.
This is not a CC bug. Android on the device is telling CC that the location is read-only.
Several CC users (for example
here) and have reported this kind of problem with upgrades, often on Samsung devices. During the upgrade Android gets confused about the permissions on the SD card and won't permit CC to access it. Often the problem is solved by uninstalling and reinstalling CC (be sure to back up the library first. See
here and
here.). In some cases the problem can be solved by asking CC to move the library off the SD card to main memory, deleting CC's private folder on the SD card, then asking CC to move the library back to the SD card. Sometimes it requires reformatting the SD card.
Another (slim) possibility is that the path to the SD card changed during the upgrade. Verify that the path stored by CC in Settings / Formats and Folders is the actual path to the library on the SD card. You can do this using a file manager, trying to navigate to the folder path component by component.