How to ignore GET parameter that’s the same as a CPT slug?

You should always preface your CPT names with something to avoid stuff like this – common naming leads to conflicts. My recommendation, so that you avoid this happening again would be to go and re-work your CPT code in your plugin/theme and use myplug_newsletter as the name of your CPT. Obviously change the myplug_ to something unique to you.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)