A while ago, we (myself and our web developer) customized our master page. We made a copy of v4.master called v4custom.master and edited it. This was at the site collection level. Just recently, a user created a form in her site (subsite) and when she tries to edit it in SPD, it gives her the error "The Master Page file 'http://sharepoint.abc.xyz/_catalogs/masterpage/v4custom.master' cannot be loaded. Attach a different Master Page or correct the problem in Code view." I tried to navigate to that location in a browser & reports 'webpage cannot be found'. Navigating in my Site Collection, I see the master pages are located in http://sharepoint.abc.xyz/_catalogs/masterpage/Forms/AllItems.aspx.
Everything else appears to be fine. Does anyone have any suggestions as to what is causing this error?