Error message in the log file:
[Server-Worker-6/WARN]: Texture minecraft:item/leather_helmet_overlay with size 1x1 limits mip level from 4 to 0
[Server-Worker-6/WARN]: minecraft:textures/atlas/blocks.png: dropping miplevel from 4 to 0, because of minimum power of two: 1
Ok, my bad! This was caused by the "Plain Leather Armor" resourcepack from Vanilla Tweaks. This can be closed.