I have Joomla 3.9.21 and RokBox 2.0.15.
The RokBox button generate an "Internal Server Error The server encountered an internal error or misconfiguration and was unable to complete your request."
When I add a syntax to my HTML, for example <a data-rokbox data-rokbox-caption="Autogenerated Thumbnail" data-rokbox-generate-thumbnail href="/live/joomla-extensions/images/rokbox2/planes_2.jpg"></a>, the editor cancel the string.
All the plugins are correctly installed and activated.
I have Gantry 4 and Quasar template.
Please would you post your URL, superuser id and pswd in the secure tab of your post and I'll have a look for you.
Tell me exactly where you used rokbox - what page? what module/article
Please search forums before posting. Please make sure your post includes the version of the CMS you are using and a link to the problem. Annotations on screenshots can also be helpful to explain problems/goals. Please use the "secure" tab for confidential information.