Hi, me and a friend have a save including factory buildings. After we researched the mouseover preview tech, my friend moused over a factory and immediately went into a Desync loop and gets dropped. We are running Factorio 0.15.4 with Factorissimo 2.1.2 and replicated the bug.

Dammit. Thanks for telling me. In the meantime, please disable mouseover preview in the settings.

Okay, I've disabled previews again for update 2.1.3. I strongly suspect this to be a vanilla issue, since other mod authors have had similar issues with multiplayer settings. Previews will come back soon, promise!