What version of SQL Developer are you using?
What does the relevant section of your TNSNAMES file?
Do you have other databases you can try to connecting to? Do they show the same error.
I can see hints elsewhere that this is a syntax error in the JDBC connection string. It isn't specific to SQL Developer or even Oracle JDBC. No one seemed to have an answer for any the questions that I found by googling.
As a workaround you could use a basic connection and extract the host, port , sid/service entries from the tnsnames.ora