UKO is the root for DNDN options, then E=May (A=Jan; B=Feb; etc.) then the F=$30 (A=$5; B=$10; C=$15; etc.)
So the UKOEF.o (or instead of .o, however your brokerage shows options) should be accurate. I just tried to pull a quote from Schwab and it said that was an invalid ticker symbol though - so I see your problem.
They may have converted it to another ticker symbol to accommodate the different strike prices beyond the $5 increments. Call your broker and ask. It is unlikely that anything "happened" to them - they are still there and your broker has all the records - so relax. They should still be there and be in tact.
There was a name change on the options from $30 and above but I would have thought they would have appeared under the new name. I don't have any that high so I can't tell what Ameritrade is doing with them. I think a broker call is needed.