I have a similar problem but I am using file geodatabases. The geodb is on a local server. I've noticed it tends to happen when I have lots of layers in the map (not sure if it is the multiple projections of different files that could be causing the issue - this is the only thing I can really put it down too). My dataframe and geodb are in the same projection.
There are no permission issues (I administer this particular local drive).
The only work around I have is to close ArcMap and reopen a blank map. Then add the layer I want to add (and maybe a few references layers). I can then edit the geodatabase no worries.
I also can't copy a layer from one map to a new blank map ... I will have the same issues with no editable layers.
Anyone else have any thoughts?? Resolutions??