By default, Arnold licenses are now checked out using the host computer name. In previous Arnold versions, licenses were checked out with the name arnold.user
This means that Arnold 18.104.22.168 and an older Arnold, like 22.214.171.124, won't share a license if you render with both versions on the same time on the same machine.
To make Arnold 6.2.1 share a license with an older Arnold, set the environment variable ARNOLD_LICENSE_CLM_USERNAME to arnold.user
ARNOLD_LICENSE_CLM_USERNAME supports the tokens <username> and <hostname>, so with Arnold 6.2.1 and later you can customize the name used to check out the license.
It could be just <username>, which would expand to to something like "blairs"
Or it could be something like arnold.<username>.<hostname>, which would expand to something like "arnold.blairs.NOVMJ07M9WM"