Hi,
When i was changing TASK (which are one year old data) which is created from Lead showing error 'Only one partner function of function category Activity Partner is allowed'
i debug and found that Partner function 00000021 Sales Prospect is automatically added in Parties involved AB.
see screen below screen details.
In Given task 4 party involved in WEBUI but in Debugging new value automatically added 000000021 Sales Prospect and its link to Activity Partner.
see below screen for more information.
where as if i open last 2 to 3 year old TASK and edit then its not showing any error message its successfully allow to change or update Task from lead.