This error can come up if there is a faulty game file or if there is overwhelming demand on the megaservers. There are a couple troubleshooting steps you can take for this error found in the help article linked below.

What do I do if I'm getting "An Unexpected internal error has occurred. Please contact customer service..." message when trying to login on PC?

Please let us know if the unexpected internal error still occurs after trying each step in the article above.