We have similar issue with accessing Workspace files exactly in DBR 18.1 ( previous DBR 17.xx  have worked even with relative paths OK ). 

import yaml
import os

def settings():
    try:
        script_dir = os.path.dirname(os.path.abspath(__file__))
    except NameError:
        script_dir = os.getcwd() 
    config_path = os.path.normpath(
        os.path.join(script_dir, "..", "config", "config.yml")
    )

at the end OSError: [Errno 5] Input/output error: '/Workspace/code/config/config.yml' [Trace ID: 00-188a04ab49de4ed3b4ab6e0e3ba17e58-b94fcf438d525fb0-00]
Looks file cannot be opened even  it's FDQN is resolved correctly
maybe some strict rules was introduced in DBR 18.xx