I have been trying to update the justified gallery plugin for a few weeks to no avail. Here is the error message I get.
An error occurred while updating Responsive Lightbox – Justified Gallery: Your Elegant Themes username or API Key is incorrect. Please update your details in the Settings > General tab. Valid credentials are required to update your theme.
Both your API and Elegant Themes (I use Divi) are correct and verified. Any suggestions as to how I might complete the plugin update?
Just try to update this version manually – the plugin files are available in your account Purchase History.
Updated Justified Gallery will include also a slightly different automatic updates handler – maybe this will solve the issue you have.