If you can not signup affiliates because error appears and notice you about something concern Rererral ID, the problem is in your Plugins probably.
Navigate to Plugins in your merchant panel and you can see few Plugins that are activated. You need to think about these plugins - one or more collide with another. E.g. you can not use "Referral ID length constraint" that checks length of refID and "Username Referral IDs" that clone refID with username. The userame will have the same length as required only 1 time from 1000 maybe... You need to deactivate one or more plugins to make it work. In this case, length plugin sohuld be deactivated.