Phaser 7800 printing with line

Hi everyone,

We purchased the Phaser 7800 at the beginning of this year. The last few months when printing SRA3 a line (vertical) appears on the side of our prints (please see below photo). We have tried cleaning the LED sensors but this didn't fix the problem. We don't get any lines on A4 prints. 

Does anyone know how I can solve this problem? It's very frustrating and is affecting the quality of our prints. 

Thanks in advance for any help :-) 

Laura