When trying to check-out a script, this error appears:

More so, while checking-in script, this error appears:

However, the check-out and check-in is successful. Meaning, the script can be seen in the check-out directory when OK is clicked in the error window. The same goes with check-in, when OK is clicked in the error window then check the update/modification in the script that was checked-in (i.e. describe a package that was modified), the update/modification is there.
WORKAROUND
None
STATUS
Waiting for for fix in a future release of Toad for Oracle.