nah

This is an issue caused by another plugin and doesn't have anything to do with ProtocolLib as far as I can tell. The plugin sends a ClientboundUpdateAttributesPacket and while sending it changes values inside a field of the packet which causes the error. You would need to report this to the plugin author, problem is the error doesn't tell us what plugin sent the invalid packet, should be a plugin that modifies entity attributes (speed, health, attack_range, attack_speed, armor, etc.).