CiviCRM_API3_Exception: "API error: DB Error: already exists"
Following on from the comments:

Sorry I don't speak Turkish but I put your comment into Bing translate and the short answer is "yes". The error happens when you have changed a relationship type (which happens automatically if you change the language) and you're using CiviCase in non-english. So the temporary options are:


Disable civicase or only use english. (Not the best option.)
Put the relationship type back to the english version. (Also not good.)
If you don't have many relationships entered yet you can delete all the relationships, delete the relationship type and recreate a new one, then update your case types, then add your relationships again.
Wait until 5.20.