I don't know why the extensions were able to be e-filed in the past, and does not allow it now.  Maybe the IRS changed their programming.

Assuming you mail the extension, I would make sure you have good documentation that the extension was mailed on time, so you have some evidence that it was done (in addition to that, I would even print out a copy of the e-file rejection, which would support that the item you mailed was the extension).