When an association from user/group/organisation/company to a CI is made, a relationship record get created. To support row level security, an entry is added to the CI's row level access field (Id 112). This functionality is used so when you create a "Supported by" or "Used by" relationship to a people organization, access to that CI is granted to people who have access to the people organization's company.
Field 112, out of the box, is 225 characters long. Each time you add a 'used by' relationship it adds the company id (as held in the group form) into the 112 field. Each id is 10 characters long and terminated with a semi colon. As a result you can only push 11 entries into the field before it fills up and gives the 306 error. This is a design limitation. It had not been envisioned that more than just a few 'used by' relationships would be created for each CI.
Need to increase the field size as we have more companies and groups, so the relationship count increases and we get this error.