There are a few reasons this could happen, there may or may not be something wrong, it is not uncommon, you can do a number of things to try to help (depending on what is happening internally with the ducks), and if it were me, if the egg were in a relatively clean place (dry leaves, straw) and the membrane intact, I would open it into a clear small dish and inspect it visually and with the sniff test and if all seemed fine I would cook it up, hard scrambled, and enjoy it.
Ducks that start up, whether brand new or restarting, and sometimes randomly, lay an odd egg like that.
Sometimes my runners start laying those after several months of daily laying. My theory is their egg systems just get tired. Once they take a rest, they are fine again.
Sometimes there are oviduct problems. Infections or just systems that don't quite work right for no understandable reason.
Sometimes the ducks are not ingesting enough calcium, or the proper calcium to phosphorus ratio, or enough D3.
Here are some possible helps. I won't call them solutions because they don't always work, or work right away.
Layer pellets (instead of maintenance ration . . . but not if you have drakes*)
Free choice oyster shell
sprinkling oyster shell into pellets (see note above*)
sprinkling oyster shell dust into water *
dandelion greens
extra vitamins
calcium gluconate in their water or moistened food* - note moistened food must be changed out before it gets moldy
If you have drakes, you might try temporarily putting them into a separate area, I mean even for a few hours, so you can give your girls a calcium party and see if they will ingest some more.
