It would usually be solved by restarting SQL Developer a few times, but it got to be rather annoying. I decided to devote a little time to trying to find a solution for this little annoyance and I believe I may have cured my SQL Developer connection woes by changing to the OCI/Thick JDBC Driver.
- In SQL Developer go to Tools > Preferences to open the Preferences menu.
- Expand the Database section and click Advanced Parameters.
- Then check the box labeled "Use OCI/Thick driver".
- Restart SQL Developer