I experienced an issue where I could not add another string override via the module. I'd fill out the two fields, hit save, and get a message that my override was added, but it didn't appear above the two that were already there, nor did it register/work in the Drupal system. I also could not delete the existing overrides.

I opened settings.php, to add my override there. I saw that the two overrides that appeared in string override's interface were identical to two added in settings.php. I removed them from settings.php, then went back to the string overrides' interface. Now, one of the test overrides I had entered appeared, and I could add another override with no problem.