Don't you have an older JSON backup with all your bookmarks that you can use to restore all bookmarks?

Thanks for your input. Certainly, I do have older json files. Unfortunately they are outdated and missing much of my recent research results, though.

Worse, since the ability to generate json files now is broken (on all of my machines), the automatic daily json files in \bookmarkbackups are all 0 kb.

What I need is a way to repair the broken places.sqlite.

You can check the places.sqlite database with this extension:

• https://addons.mozilla.org/firefox/addon/places-maintenance/

Good advice. Never thought of such an add-on. If required, I'll come back for support. Many thanks at any rate.

Continuation

1. The places maintenance add-on reported no errors and no corrections in the deficient places file.

2. But still, when I sync a fresh profile with an existing profile that contains the deficient places file, it acquires the same deficiency.

3. Only when I use a different bookmarks json file to restore bookmarks, the error is gone.

Something seems to be wrong in the bookmarks contents. Too long entries? Forbidden characters? Javascript bookmarklets? Bookmarks of my android device? Whatsoever: Is there also a tool to check json files?

How many devices / machines do you have on the same Sync account?

Are you having that same or a similar problem with any other devices / machines?

You might want using Unlink this device in Options > Sync on the machine where that problem exists, to see if Sync might be related to your issue.

4 desktop machines and 1 android phone are linked to the sync account.

Yes, the inability to generate json or html files spreads to all synced desktop machines. For test purposes, I added a 5th machine: It GOT the same prob.

When I restore bookmarks with a two months old automatic json backup, the error is healed, and healing spreads to all linked devices. (Unfortunately, this backup is outdated).

What kind of error may have turned the actual bookmarks data base deficient?

• JSON Formatter & Validator: http://jsonformatter.curiousconcept.com/

I don't understand why you have a two month old JSON backup file; Firefox creates a new JSON backup file every day Firefox is used for a total of ten backups (the default setting) - on a PC that you use daily, you would have the 9 previous days backup files plus the current day backup.
Have you looked at all five of those desktop PC's for a newer date backup than two months old?
On a PC that you use Firefox regularly once a week, you should have a backup going back over the last 9 weeks. So hopefully you'll find a backup file that is a lot newer than 2 months old.

The first thing to do is to Unlink this device on all desktop PC's and also on the Android phone - see if this support article helps you with Unlink ... on Andoid. I don't have an Android device and can't help you there.
https://support.mozilla.org/en-US/kb/firefox-mobile-android-tutorials
"Stop the madness" by stopping Sync from perpetrating this problem to all of your devices as you fix one device.

That will help you fix one PC without perpetrating errors on the other devices back to each device as you fix each one separately, in case that turns out what you need to do. I'm not going to offer advice on fixing the base issue or the original cause. I'll leave that to cor-el.
Quite honestly, although I have played with Sync - starting when Sync was first offered as the alpha level Weave service as an extension - on multiple "devices" with three different Sync accounts - total of 10 "devices" separately using those 3 accounts - 3 devices on the 1st account, 2 on the 2nd account, and 5 one the the 3rd account - I still don't have a full picture on how all facets of Sync operate.

Then, when you are done doing the fixing, get a new Recovery Key for that Sync account to wipe all your data from the Sync server so you are starting fresh, or set up a new Sync account for further use of Sync. services.

> why you have a two months old JSON backup file

Because prior to major changes in my bookmarks, I manually save a copy of an automatic backup file (and of places.sqlite) to a user created folder. Alas, 2 months ...

> Hopefully you'll find a backup file that is a lot newer than 2 months old.

I won't, since all machines got the same error: they have generated json files with 0 kb for quite a while.

> I still don't have a full picture on how all facets of Sync operate.

Formerly, I used to sync a bunch of virtual machines with several accounts, but recently confined to my physical machines only. Sync was (and is) too unpredictable.

> new Recovery Key for that Sync account to wipe all your data from the Sync server

Definitely good idea. I will do so. Many thanks for dealing with my problem so thoroughly.