WebdriverIO released v5 around Christmas, see also (https://webdriver.io/blog/2018/12/19/webdriverio-v5-released.html).
To make this reporter usable with the new version there are some small adjustments necessary. There are some minor changes that need to be made. You can find a general overview
(https://webdriver.io/docs/customreporter.html).
Most important ones to point out provided on this link:
webdriverio-community/wdio-mochawesome-reporter#36
Thank you a lot!