Pro Sites has broken my site after moving server

Hi There,
Today I moved my site from my shared host (with dedicated IP) to a VPS server. The move was done easily using my hosts migration plugin, and all files and cpanel settings are identical to that of my shared host.

I've set up DNS correctly on my VPS and changed the nameservers of my domain to point at it. Everything works as it should and my site "shows" in the right place (i.e. DNS lookup shows it's the correct IP address and nameservers, also if I place a index.html file in the root of my server and navigate to it, it shows).

The issue I have is that my site was giving me a 500 server error. I looked up the error logs and get the following:

[Wed Dec 30 11:30:26 2015] [error] [client xx.xxx.xxx.xxx] SoftException in Application.cpp:256: File "/.../.../.../xxx/admin-ajax.php" is writeable by group, referer: http://www.mydomain.com/wp-admin/network/settings.php?page=domainmapping_options

(blanked out actual urls for security). I have traced the issue back to ProSites - i.e. if I disable prosites plugin, my site shows, if I enable it, it doesn't work (I've had to disable it via ftp).

Can you please help me figure this out ASAP as I have live clients on my site and need this resolved. I have granted support access in advance of this post.

Thanks