DVD Recorders have several different modes and support different media types. Each maker supports different formats also.
For example, Sony supports DVD-R and +R formats. Panasonic supports DVD-R and RAM, but not +R. Toshiba, Pioneer, JVC, etc also support different variations, it just depends on which one they get royalties from
On top of that, there are DVD-VR, DVD+VR and DVD-Video formats. DVD-RAM only supports VR mode. DVD+R and +RW support Video Mode. DVD-R and DVD-RW support VR and Video modes.
VR mode is used when one wants to add more recordings, edit, etc. However, unless the disc is closed, it can't be used. Video mode is better for compatibilty, but can't always be added to and can't be edited, etc.
Honestly, it is all a real mess.
Popcorn does not support DVD recorded in VR mode. It simiply will not access the files. Popcorn will support DVD recorded in Video mode. DVD-VR discs seem to be OK. However, DVD+VR discs are different and obviously don't work. This is why you get the error.
I don't know if that is something that can be fixed in Popcorn or not. DVD+VR may just be blocked like -VR are (though I think it will work like Toast since they are in Video mode).
Toast on the other hand supports all these formats and is really the app of chose for DVD-VR and DVD+VR. It has the UDF 2.0 engine built in to read all the VR discs, and also access the disc differently.
So, in your case, you can use Toast or you can record to DVD-R using Video mode, not VR mode.
Hope that clears it up a little.