I don't use this app but I see it says you can choose where to back up, so there must be somewhere to set the destination. It is trying to back up to a strange address at the moment so that may be the problem.
Have you checked to make sure your phone hasn't run out of storage space?
The error message doesn't really give any useful info, but it could be lack of storage space, if not you could try saving it somewhere else if possible (It seems to suggest you can save it wherever you like)
The problem is not the space and not the path of the file
I think the problem with the file storage is the same
PgtBackup.json
More about file *.json with JavaScript android
Code source https://www.tutorialspoint.com/android/android_json_parser.htm