Can't develop my site because of this -Warning: Cannot modify header information

On the weekend I went online to start developing my website. The theme is
Aegaeus purchased on 23rd April 2015. The following warning message appears when when I login, change and when attempting to load images. This is the first time I’ve attempted to use this new template. I purchased it via – Anniversary Bundle 2015 Support and paid to have Mojo Install it on 7th June 2015.

My web host HostGator have looked into it and cannot find a solution it seems to be something deeper at the Template level.

Connection error! please try again….
Warning: Cannot modify header information – headers already sent by (output started at /home3/ab1276/public_html/wp-content/plugins/screets-chat/core/fn.init.php:453) in /home3/ab1276/public_html/wp-admin/async-upload.php on line 35

This notice appears at the top of the page, under the black toolbar header and above the page content.

I have asked Mojo for help and they suggested that I check with the seller.

Basically this is currently useless because I can’t load anything. This is going to be an Art website so it will be fill with images and video links.

Please help!

Kind regards,

Sonia