Of course a lot of emails were send out and members replied and told us they didn't ask for them, many did not understand why they were added or did not even know about the conversation. People even reported us as spammers because of many got over 20 mails send to their address. This was a chaotic moment and we decided to recover from a backup of the day before, since there wasn't enough time to figure out what was happening.
Here's a screen of the conversation info:
* We are running version 1.5.6
* No add-ons installed related to conversations, see this list:
* About 8M posts and also many PMs were imported from vBulletin 3.8 in 2015 around september, no issues since then, until March 5 2016 this happened.
* a few days before, I (as admin) have left this and some other conversations, to clean up my inbox a bit
* We were not working at the forums when this happened and no server error can be found.
If you need more information regarding this, please let me know. I can also provide login info.
We really need to know why this happened since we can not afford this to happen again.