In the Main Coordinate Convertor on the AIJ Toolbar (the protractor icon), go to Preferences > "Open custom observatories folder", and let us know the folder location.
Do you know if you have write permissions for the above folder?
After you click OK in the error message, does DP still open as normal? If it does, try going into DP CC and turn off "Use custom observatory list". Then close DP CC and DP and reopen DP. Does the error message show?
Finally, Are you using Windows, Mac, or Linux, and what is your AIJ installation folder?
We just created 6.0.2.00 that has some other updates. We also included a potential fix for this issue. Please update to 6.0.2.00. If the problem still happens, please provide us the information asked for above.
Also, if it still happens, please copy and paste the contents of the log file at the following location in a reply.
%LOCALAPPDATA%\AstroImageJ\Logs\aij.log