This can be frustrating, and the solution is not obvious. What is an unknown node anyway? And what should you do when this happens?
First, try saving your work as a different file type. Maya offers two kinds - .mb (small files) and .ma (larger files). If Maya won't save your work as a different file type, proceed to step two.
Under the file tab in Maya, scroll down to find "optimize scene size" (see screen grab above). Open up the options box. Make sure that "remove unknown nodes" is selected and the box is ticked. Now click "optimize". Maya should now allow you to save your work.
If this still doesn't work, try Step Three.
Note that there are three different types of unknown nodes that can prevent Maya re-saving a scene in a different format. They are: Unknown nodes, Unknown transforms and Unknown dag nodes. The second two are uncommon. To delete all three types, open up the Script Editor in Maya and type
delete `ls -type unknown -type unknownDag -type unknownTransform`
If this still doesn't work, start at the beginning and try again. It will work in the end!
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