We are also getting the same issue in Sitefinity 7.2. The way to reproduce this is
Step 1. ) Navigate to Administration->Settings->Advanced->Data->set Enable Data Caching to false
Step 2 .) IISReset
Step 3.) Navigate to any ecommerce front end pages which will be calling any manager like var mgr = Catalogmanager.GetManager(); mgr.GetProducts().FirstOrDefault();
Step 4.) Ideally the exception can occur here. If not then repeat Step 1 and set Enable Data Caching to empty value.
Step 5.) IISReset
By now you should get the error most likely in the index page itself or the page where you are calling the instance of the CatalogManger.
Hope this helps.