Some addon is spamming the chat
 

Any idea what causes this? I assumed it's LibHistoire because there was no spamming when it was off, but
sirinsidiator told me it's not it.

Master Merchant has Debug routines used during development. If you upgraded from 2.x to 3.0 the highest setting was 5 in the previous version. Therefore you would not see those messages. Most likely you set verbose to 6 or higher. Verbose is no longer really used. Set it back to 4, which is the default setting. I will be removing that in a future version as I consider all notifications important.