I feel sure, from hints I've read, that this issue is not new. But, I can't seem to find someone's post as to how they fixed it.
All of a sudden, after months and months of working the same project (smoothly, without a hitch), this began happening.
Before I post the output, let me just say that to try fixing it, I've done scans of the file system for all manner of Malware and Viruses - NONE found. And, an important fact is that I have NOT changed my system in any way other than to continue writing and developing sketches. No new libraries nor boards nor even new ANYTHING. I just woke up one day and this occurs. I also would like to mention that I have performed a restore of the ARDUINO directory from a professional backup (Backup4All) as well as from full Arduino directory raw backup files I routinely save. Adafruit's json info is correct in Preferences and I can type the URL directly into a browser and fetch their boards file but otherwise, only the native Arduino boards appear in Boards Manager (with a few token others). I can keep doing all the 'try this, and try that' experiments of trial & error but I'm appealing to someone who has specific experience with this who, hopefully, will read this and share the specific solution.
This is the IDE output following any attempt to use: Tools > Board > Boards Manager.
IDE responds properly with : "Downloading platforms Index" and then this is the result:
(((( The output repeats itself to about 400 lines of this.. I edited it down to this sample to remain within the 9000 char limit of a post.)))
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\Adafruit_Trinket_USB: C:\Documents and Settings\Bill\My Documents\Arduino\libraries\Adafruit_Trinket_USB
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\arduinoFFT: Missing 'maintainer' from library
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\reorder_table_creator: C:\Documents and Settings\Bill\My Documents\Arduino\libraries\reorder_table_creator
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\Adafruit_Trinket_USB: C:\Documents and Settings\Bill\My Documents\Arduino\libraries\Adafruit_Trinket_USB
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\arduinoFFT: Missing 'maintainer' from library
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\reorder_table_creator: C:\Documents and Settings\Bill\My Documents\Arduino\libraries\reorder_table_creator
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\Adafruit_Trinket_USB: C:\Documents and Settings\Bill\My Documents\Arduino\libraries\Adafruit_Trinket_USB
Invalid library found in C:\Documents and Settings\Bill\My Documents\Arduino\libraries\arduinoFFT:
I always like to follow up with a solution to my problem if I happen to resolve it before someone, having had the same issue, gets to respond. A number of people have viewed the issue and this will help any one of them wondering the same question/problem solution.
After much to do, I dragged out a clone of my C:/ drive and systematically (no pun intended) copied over Arduino-related directories. I did this directory-by-directory so that if a solution was forthcoming, I'd not have the nagging question in my mind as to the cause. So, with each restoration of a given directory (Adafruit drivers, Arduino libraries, sketches, on and on), after each one -- I'd boot up, start the IDE and see if the boards manager was back in business. Then, BINGO... with the copying of the ARDUINO-15 directory of files (a good 2-4 minutes copying), everything is back to normal. What corrupted this? Since I have a very reliable power supply backup system here so that my system doesn't even blink when utility companies do, I know power instability was not the issue. I suspect WINDOWS and I suspect that the random, out-of-the-blue come 'THE DREADED BLUE SCREEN', is the cause of corruption. These rare and unpredictable events became fairly frequent with the re-activation of a back up program's scheduler to backup my Arduino files once every 2-3 hours. Apparently, it was during one of these events that the planets lined up, the system crashed, and corruption of the ARDUINO-15 file(s) occurred. In any case, maintaining a good copy of the ARDUINO-15 directory is now a regular part of my daily shutdown routine. I'll keep 5 to 10 in a FIFO schema.
I hope this helps someone cut to the chase and think Arduino-15 (in the Local Settings/Application data folder)
Bear in mind that the IDE is written in Java. Java is not the most stable platform on the planet and the IDE consistently crashes/disappears when you have more that 5 or 6 instances working (on my systems anyhow - all of them). So I wouldn't put it past that reality to be the source of otherwise unknown bugs/crashes/corruption.
Govner:
In any case, maintaining a good copy of the ARDUINO-15 directory is now a regular part of my daily shutdown routine. I'll keep 5 to 10 in a FIFO schema.
There's really no need for that. The only irreplaceable file in that folder is preferences.txt and the Arduino IDE has so few preferences it would only take a couple of minutes to reproduce your old settings if for some reason you lost that folder or it was so corrupted you had to delete it completely. Everything else in that folder is always available from the internet. It might take a while to re-install it all as some of the toolchains are pretty hefty but it's an automated process that would be a rare occurrence so it wouldn't be the end of the world. To resolve a problem like you encountered you would only need to delete the specific Adafruit boards index files from the root of the Arduino15 folder: package_adafruit_index.json package_adafruit_index.json.sig, package_adafruit_index.json.sig.tmp.
Certainly one of those files getting corrupted should not cause this problem. The Arduino IDE should be smart enough to delete them and download a fresh copy from the Internet. I know the Arduino developers did some work towards that goal but either the IDE version you're using doesn't have that work or else maybe the problem wasn't completely solved.