After upgrading from 2.0.7 to 2.1, none of my configurable products will save on edit. In the browser console I can see there is an ajax call to the Product/Validate controller that returns a 404 response (ex.
admin/catalog/product/validate/id/269/type/configurable/store/0/set/4/?isAjax=true). This failure stops the save step from ever occurring.
Creating new configurable products works fine, editing simple products works fine (for example
admin/catalog/product/validate/id/270/type/simple/store/0/set/4/?isAjax=true resolves correctly).
Accessing the validate URL directly in the address bar also works (at least to resolve to a 200 response).
I've tried deleting and completely re-installing the vendor directory via composer, after clearing the composer cache so new files were pulled.
Tried recompiling, all the usual cache clearing, re-indexing.
Does anyone have any ideas on what to try to possibly resolve this? I'm baffled.
------- Update -----------
The OS distro that I am running this on is CentOS 6.7. Interestingly, I have a Windows 10 development installation on XAMPP that I upgraded to 2.1 and all seems to work there. I also tried a clean install of 2.1 in a new directory on CentOS, but that also failed in the exact same way.
I find it interesting that a GET request (pasted into browser address bar) returns a 200 response, but a POST request does not. To me that signals some sort of failure at the routing level to recognize the URL pattern via POST, but I'm not quite sure how to track it down. Is it related to this particular Linux distro? No idea yet.