When a user attempts to open the Join Dialog or attempts to Duplicate the sheet, an error 500 is generated in the background of the web-page. Additionally, the Flip-Sheet button does not activate in the header.
Here is an example URL that generates the 500 error:
This is a known bug affecting Datameer 4.4.
To resolve this issue, upgrade to Datameer 4.5.5 or later.