So what happened?
The problem was NOT the unlink function that some people reported. That was a “red herring” (looks like the real thing, but it is not).
The problem had to do with how Pro Pack and Store Pages are connected. While working on 3.8.15 I noticed a bug on how those get activated & setup when a license is entered. It would not ALWAYS load the Store Pages or Pro Pack features early enough. I fixed that bug.
Problem is that not everyone has both licenses. My development system and primary test system do have those (I forgot to reset my test system before running 3.8.15 testing). Thus I never saw the problem. In loading those add-on modules earlier to fix the first bug, it caused another bug that basically was saying “HEY, that thing you just tried to activate does not exist” and would crash immediately.
So, now I make sure those things (Pro Pack module or Store Pages module) exist BEFORE activating them. Like I should have done in the first place.
Side note:
This fix also caused the BASE PLUGIN (Store Locator Plus, not the add-on packs) update modules to start firing off again. Some installs would run into a problem and “hey I’ve got a new version now” code would never run. Now the update function runs. That means most installs should be happier. It updates the store locator table properly. One of the changes made 6 months ago was adding a couple of indexes to the locations table. That should make installs with more than 100 locations run a LOT faster. That is assuming your MySQL install can handle 4 more index keys. If not it simply puts out a warning on activation and moves on without the indexes. No harm, just the lookups will not be any faster with the patch.
HOWEVER, some sites may run into unknown problems now that the update function is running again. If you see something new, please start a new support forum thread and I will look at it ASAP.