This has happened to me a few times recently, with different tasks. When dragging a task to a new column (possibly always to/from a "milestone" column), it fails with a popup error like this:
Unhandled Exception ("AphrontDuplicateKeyQueryException")
#1062: Duplicate entry 'PHID-PROJ-4y5gw5adxnh7eb5t6mqq-PHID-PCOL-plsngegjuwikmxt6opfn-PH' for key 'boardPHID'
An example you can see right now is in Team-Practices. I have been trying to drag T141138 from the "This-Week" column to the "General Backlog" column. I can leave this test case there for a few days, in case it helps with debugging.