http://www.techradar.com/us/reviews/audio-visual/dvd-recorders/panasonic-dmr-ex773-687389/review was unclear if it took the -R media too.

Here my recorder does NOT format the DVD media but uses some "dubbing" method from HDD to DVD which I won't enter into any semantic debate or tech discussion over inaccuracies in how they describe or implement recording.

HOWEVER I would try -RW media if it supports it after you try a CD/DVD lens cleaner. As in all optical recorders we see 2 to some year life spans. This unit may be about 4 years old and repairs always seem more than replacement costs.
Bob