The Service Desk install has been configured so that when the ticket status is a certain value, certain fields will lock and be inaccessible for end users to update
Finding that for html_description fields, the field is editable if the field was included to be locked for selected statuses. If the ticket is saved with that status and description update, the web UI will present with an error
Release: 17.4 RU2 - RU4
This is a known problem with the web UI for html_description
This functionality has been addressed in 17.4 RU5. See entry 36477139 in the 17.4 RU5 release notes
A common mistake with this scenario is to add only the "description" field to the Status Transition settings. You must add "html_description" field. The "description" field can be omitted.