Gitlab, fail of yarn when migrating to 10.0.x

For once, the Gitlab upgrade process from 9.5.8 to 10.0.x was not totally smooth on my end. I encountered the following error:

error "webpack#ajv" not installed
error "webpack#ajv-keywords" not installed
error Found 2 errors.
Error: You have unmet dependencies. (`yarn check` command failed)

And installing ajv manually did the trick:

sudo -u git -H yarn add ajv@^4.0.0

In case this helps someone out there!