[Xposed Framework Modules] DrPhant0m:
I created the following folder, and copied my previois SnapPrefs folder contents there: /storage/BFF8-1BE6/Android/data/com.marz.snapprefs/

"BFF8-1BE6" is the name of my SD card that Marshmallow assigned when I was forced to format the card. I know it's weird --- I didn't have a choice. This directory is similar to the one I had to use to fix Titanium Backup with a similar Marshmallow-related problem.

I then used FX file explorer to copy the path to the preference XML file so I'd have no typos.

Here is an excerpt from the log. It shows "Permission Denied" to the directory I've entered in the XML file.

12-30 00:36:25.006 I/Xposed (17081): ----------------------- SNAPPREFS/Sent Snap ------------------------
12-30 00:36:25.006 I/Xposed (17081): SnapPrefs: Saving sent snap
12-30 00:36:25.011 I/Xposed (17081): SnapPrefs: We have the Context
12-30 00:36:25.011 I/Xposed (17081): SnapPrefs: We have the MediaBryo
12-30 00:36:25.013 I/Xposed (17081): SnapPrefs: We have the filename 2015-12-30_00-36-25
12-30 00:36:25.013 I/Xposed (17081): SnapPrefs: The sent snap is an Image
12-30 00:36:25.078 I/Xposed (17081): SnapPrefs: Exception while saving an image
12-30 00:36:25.084 E/Xposed (17081): java.io.FileNotFoundException: /storage/BFF8-1BE6/Android/data/com.marz.snapprefs/SnapPrefs/SentSnaps/2015-12-30_00-36-25.jpg: open failed: EACCES (Permission denied)

So... then I modified the Prefs file and added "/fakefolder" to create a situation where Snapprefs is pointed to a folder that does not exist, to test your theory. Instead of "Permission Denied" I got "Failed to create directory..." Note: I did not create the "fakefolder" folder... I just added it into the XML file to point it to an invalid location.

12-30 05:55:32.632 I/Xposed (30960): ----------------------- SNAPPREFS/Sent Snap ------------------------
12-30 05:55:32.633 I/Xposed (30960): SnapPrefs: Saving sent snap
12-30 05:55:32.633 I/Xposed (30960): SnapPrefs: We have the Context
12-30 05:55:32.633 I/Xposed (30960): SnapPrefs: We have the MediaBryo
12-30 05:55:32.642 I/Xposed (30960): SnapPrefs: We have the filename 2015-12-30_05-55-32
12-30 05:55:32.642 I/Xposed (30960): SnapPrefs: The sent snap is an Image
12-30 05:55:32.673 E/Xposed (30960): java.io.IOException: Failed to create directory /storage/BFF8-1BE6/Android/data/com.marz.snapprefs/SnapPrefs/fakefolder/SentSnaps

I can add more of the logs if necessary (they're REALLY long, even for a few seconds of capture)... but this seems sufficient to show a contrast between "Permission Denied" and "Directory does not exist." I should also add that I get "failed to create directory" when I point it to another folder that I created on SD, that was not in /storage/BFF8-1BE6/Android/data/com.marz.snapprefs/. For that, I created a "1" folder in the root of my SD card, and put /storage/BFF8-1BE6/1 in the XML. I believe it failed because, as the dev of Titanium Backup stated... "Google uses a very clean security-solution by giving apps that request write access to external SD, permission only to their own folder – not to the entire card"

Is this enough to prove that I have a valid directory entered, and that SnapPrefs isn't able to access my SD card correctly in Marshmallow? To me, it seems that I've given valid logic... but I'd rather be proven wrong with instructions on how to get it to work.

(Manually-added CODE tags with tapatalk... hope it works)