When editing a many-to-many related list, loading the list of "Available" records fails when there is already a large set of records "Selected", making users unable to modify certain many-to-many related lists.
M2MList ajax call fails with a large set of m2m_selected_ids values, generating the error:
This problem is under review and targeted to be fixed in a future release. To receive notifications when more information becomes available, subscribe to this Known Error article by clicking the Subscribe button at the top right of this form.
By default the Transaction Quota Rule (/nav_to.do?uri=sysrule_quota.do?sys_id=52b8381637d86700b097339b34990ec6) would be 15sec, the workaround would be to modify the Transaction Quota Rules to 20sec, or possibly higher, but not exceeding 35sec.