Second time in last 10 minutes I run into "serialization is required for constructing a EntityId and must be a string" error when adding a statement to SDC.
In both cases the item I am picking just went through merge operation with other item. I paste new item ID in the window, pick the item from suggestion list and get the error. In both cases adding English label to the item and purging the file page resolved the issue (not sure if the first step is necessary).
Might be related to T143249.