I'm trying to register a new SharePoint App using _layouts/15/appregnew.aspx due to a ClientID change but I'm receiving

Sorry, something went wrong

Which is not very useful! I've confirmed the Service Principal doesn't exist using Get-MsolServicePrincipal -AppPrincipalID xxxxxxxxxxxx but I'm now out of ideas.

Anything else I should be checking?

Sincerely,
//Adam

Hi adam_j_bradley,

Do you mean you have changed the ClientId generated on the page?

If so, the ClientId should be unique and lowercase:
http://msdn.microsoft.com/en-us/library/office/jj687469(v=office.15).aspx

As this is for SharePoint online, people in the Office 365 online forum should be more professional on this, I would suggest you to go to that forum for more information:
Office 365 forum:
http://community.office365.com/en-us/forums/default.aspx

Thanks,

Silly me. Two totally different AppId's. One for SPO and the other for WAAD (:
Thanks, working now..

//Adam