I found this page [0] (in French). I don't know how reliable this website (or my French, for that matter) is, but it seems like its a group of activists illegally squatting, damaging the property (at least changing the locks) and causing some public disturbances in the street, and the police were having a difficult time catching them. This email account was linked to the organization's Twitter account, and from there they were able to put together enough information to arrest.

[0] https://paris-luttes.info/recit-policier-de-sainte-marthe-15...