Known issue. Been like this for a month or so. On dullta.com most non-DL flight awards are blocked and won't appear (at least KE, MU, CZ, FM, CI, GA, SU, etc. routes I've tried with plenty of availability). Used to work in the DL app, but now that is also more and more often not working. Even when it does, O&D searches usually won't work. Have to do segment by segment to see availability, it won't see most connections that before it would always show.
For phone agents also buggy and need to reload and search 20+ times to have flights show.
Seems that DL is in the process of rolling out SkyTeam (non-DL) award blocking, or going to a segment-by-segment based pricing model, or some other such "enhancement"... or it could be just an outage in connecting to other SkyTeam airlines for a long time that has went unresolved.
Best case scenario it's just gross incompetence by DL of not maintaining and monitoring its systems and an extensive partial outage. But the real risk is that it's another "enhancement" in the works, that non-DL awards will be priced so outrageously that DL is afraid to even show it online and will force you to call and book, or block some outright. Pure speculation, but based on previous tricks that DL has done countless times recently, it's certainly in the realm of possibility.
I am surprised how come this hasn't gotten more attention FT here recently.