In Portal clients get "Failed to update request participants field, errors:"

Occasionally our customers get the error:

Failed to update request participants field, errors:

when trying to Share a ticket with their colleagues via the Customer Portal.

Sometimes I, as an Agent, see the same error when I try to Share from the portal, and sometimes I can Share from the portal even though the client cannot.
I can always add the user from the Agent View to the Request Participants field.

Looking at the atlassian-jira.log there are no obvious related logs, but I can supply a log if that would be helpful.

I have reviewed
JSDSERVER-1786
JSDSERVER-3733
JSDSERVER-5112
and
https://confluence.atlassian.com/jirakb/customers-are-unable-to-add-participants-to-a-service-desk-request-777027039.html
but these are not related base on the reproduction and work arounds.

Reproduction steps:
1. Customer creates ticket
2. Right away try to Share with other user (ticket has not yet been assigned or touched in any way by our agents)
3. Get the error seen in the screen shot attached

This is really a bug but the system only lets me submit suggestions, so I suggest you fix this bug.