On the old FP Open, I was able to solve SD card problems (apps couldn’t write on it) with SDfix from the Play Store.
On the current version, this doesn’t work. There is a Marshmallow version of that app but it has no effect. With this problem, I cannot have Titanium Backup write on my SD card and subsequently don’t want to run an update.
Related question: If I reformat the SD card as internal storage (which would supposedly fix the problem), can I write on it from my PC?