Can't activate/deactivate/resolve accounts/cases etc Suddenly

Good Afternoon,

I've been battling a new issue today and having googled immensely, I just can't find anything to point me in the right direction. I would greatly appreciate any assistance, seems to be a unique problem:(

Using Chrome and F12, i've been able to capture this error: 
POST http://<url>/<organization/XRMServices/2011/Organization.svc/web 404 (Not Found)

If I manually browse to that URL, i get a 404

If I remove /web, i get a standard service page

Could anyone point me in the right direction here? best i can tell is that these calls are native to CRM and not some addon functionality (I can't find any custom work either, other than the activityfeeds plugins, which interact with account and case creation/resolution (i think))

Possibly relevant points

• this is a CRM that was upgraded to 2013 about 1 week ago from 2011, but the issue wasn't present until yesterday
• CRM Email Router is still on version 2011, but I don't think we use it