Topic on Manual talk:Common errors and symptoms

It appears that when read only mode ($wgReadOnly) is active, you might also see this error.

When you see this error message on pages or for some files in your file list, you can try to remove $wgReadOnly from your LocalSettings.php.

Feel free to add that paragraph

Thank you for the quick reply.

I do not have permission to edit semi-protected pages. Perhaps someone who does have permission can add the following underneath the existing paragraph of 'Unable to save thumbnail to destination':

This error can also occur when read only mode ($wgReadOnly) has been set in LocalSettings.php. You can try to remove $wgReadOnly to see if that solves the issue for you.