Mostly only returns error 500 and oAuth URL

This is the place to get support, so you have that covered.

As to your issues, when you get 500 errors, is there any message that is shown on those pages? You mention "Mostly only returns error 500 and oAuth URL". Is the oAuth URL an error, or something else that you're seeing? Any information you can provide on what exactly is happening will greatly help us help you.

The main things I'd recommend with the information you've provided is to check the LinkedIn Application you created in their developer area. Ensure that the "Website URL" is set properly to the page you are testing from. That URL should look like either:
yoursite.com
www.yoursite.com
You should use the same subdomain (www or non-www) when testing on your site as well. If your URLs are not correct, then when a redirect is performed after logging in, LinkedIn will think something 'bad' is being done and will throw an error.

I hope that helps, but again, if you have any information on errors your seeing, pages we can test on, or anything else you've observed, that would help.

Thanks,
Alex