I didn't solve it, but in case someone in the world needs a solutions here it is:
Install malisis door mod 
Posts by nymeriah
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Hi guys!
I installed the version 1.12.2 and created the server and put some mods. Everything works perfectly except some objects that appear with the classic missing texture cube, purple and black cube. It happens with some kind of wood.
What kind of mod can bug this things? Maybe Rustic or Dynamic trees? We have no texture pack installed...
Should I reinstall something?
PD: In the single player mode, they work perfectly, it's something related with the server in Aternos.