Well, something is interferring with the settings page.
Javascript code is used on the client-side to encode settings and generate checksum.
If the plugin (on the srverside) cant’t verify checksum it reports error.
For this to work you need to have error free settings page.
The plugin is working on 300 000 websites without problems.
Have you tested with some default WP theme like Twenty Seventeen or something?
Default WP theme with all plugins disabled shoud work normally.
If you still have issues then this might be due to external caching, browser issues (invalid cookies) or web server settings.
From the error message you posted it seems something is interfering with the javascript code / syntax highlighter used by Ad Inserter.
Similar issues in other similar cases were caused by CDN or optimizing plugins that were modifying Javascript code, but due to various bugs they in fact created invalid code.
Maybe you can try to use old version of the plugin to disable syntax highlighting and then try the latest version.