@SarahT: It would be nice if the problem was as simple as a physical sync with a computer. Unfortunately, it's not.
I sync every day, and sync with a physical cable to the computer at least 3 times a week. The problem persisted until I followed the iExplorer procedure.
The "Unable to Purchase" popup doesn't occur every time you purchase/download an app. The symptoms that were consistent with this problem for me were:
- Apps that I were on my device turning Grey for a brief amount of time with a slot near the bottom for the progress bar (as if the apps was being updated/downloaded), then the icon would reappear, but the slot would stay for a second or two more, then the icons would fully restore.
- The device would freeze for about 10 seconds a few seconds after being turned on (woke up). Ex: Wake up the device with the power button, watch the "slide to unlock" animation. After a few seconds, that animation would stop for about 10 seconds, then start up again (assuming the screen stayed on. many times it would turn off again (go back to sleep) before unfreezing).
Those two symptoms occurred pretty much every time I attempted to wake my iPhone and iPad. After applying the iExplorer fix (deleting the 'downloads.28.sqlitedb' file and the folders in the Downloads folder), my devices have been performing as they did when I first got them. No freezing, no weird icon greying/looking like they are being updated when they were not).
I can't say that your situation is the same as mine or others here, but it's likely that the iExplorer fix will clear up your situation as well. Even if not, the Downloads folder probably shouldn't have anything in it other than the manifest.plist file and the downloads.28.sqlitedb file. (deleting the sqlitedb file fixes the problem because it's corrupted.) I say this because looking at my Downloads folder two weeks after applying the iExplorer fix, I still only see the manifest.plist and downloads.28.sqlitedb files. I've downloaded several apps and songs since I applied the fix.
If your Downloads folder has anything other than those two files (manifest.plst, downloads.28.sqlitedb), it's likely that the sqlitedb file is corrupt and needs to be deleted (it will be recreated by iOS) as well as any folders (assuming you are not currently downloading an app).