This may or may not help, but it sounds like at some point you may have accidentally set your "browser" as the default app for the file type that you are trying to open through ServiiGo. I've used the same setup with Android 6.0.1 on my family's S5's and the ServiiGo app without the problem you are describing. But I have seen that problem with other file types and other programs on Android and had to clear the defaults to correct it.
I would suggest clearing the defaults for the browser that opens when you try to play a file. You could also clear the defaults for the VLC app as well as the ServiiGo app if that doesn't resolve the problem. Afterward, setting ServiioGo to "force external player" should give you the option to pick a player app such as VLC and set that app as the default app when you next try to play a file.
https://www.google.com/search?q=S5+clear+default+appsAnother thing to try if the above doesn't work is either clearing the ServiiGo app data or uninstalling the ServiiGo app to clear it. Then reinstall and trying again.
https://www.google.com/search?q=S5+clear+app+dataLet us know if that helps.