That looks like it is a popup message displaying that error. Are you using IIS for your web host? Also, what are you doing when it pops up? This could very well be a SSL settings issue. I use IIS, and ran into similar problems on multiple websites until I was able to figure out how to properly set up my certificates. SourceBans++ is also not yet PHP 7.x-friendly, so it has a lot of issues that appear to be being fixed by the developers. This is going to make it a bit harder to debug, so keep that in mind.
What version of PHP and MySQL Server are you using? If you are using IIS, what version? How did you set up SSL?