After some testing, the problem is in fact related to Biomes'O'Plenty. If I de-activate it on the client, it joins the server just fine. I can also see on the server startup messages that Biomes'O'Plenty is not beeing loaded... what am I doing wrong? I also tried downloading the mod from the minecraftforums page and manually adding it to the jar, but it still won't recognize the mod as beeing loaded.
Disregard, I must be the person with the lowest IQ on this solar system. I was installing Biomes'O'Plenty the old way, copy-pasting the files from the zip straight into the server jar... All I had to do was go to the mods folder and remove the .disabled from the zip. Voilá.