It appears that the "convert to HTML" option is not longer available in Dashboards from 6.5.2. I am also getting a "client not authorized" message when I try to edit existing HTML dashboards from previous versions. Is this a feature that is no longer available, and if it is what could be causing this issue?
Splunk Version 6.5 onward the "Convert to HTML" option has been shifted to Button with three dots right next to Export button. It is no longer available under the Edit button as mentioned in the dev.splunk.com. I have submitted a document feedback for it to be updated. The same is also applicable to Edit Permissions and Clone options that have moved from Edit button to the new Button with three dots.
Requesting you to provide a document feedback on docs.splunk.com in case you come across an error or incorrect information.