Hi,there.
reproduce:
1.when I add a user on server and input it on PC Anki,first hit sync button,everything was Ok.Then I tried to add card with plain text or pictures(media) and synced,the following dialog popped out.

I guess It cannot sync after operations of add/deleteing cards/decks and learning trace.

2.Then I go to tools-->preferences-->scheduling,select Anki 2.1 scheduler checkbox and closes.And I hit sync button,It prompts me to select upload and download .Then I hit download from ankiweb,It seems that works and the effect is equivalent to upload.Yet
upload button will show the check database dialog.

3.I wait for several minutes until logs like collection thread running close was printed on server window,I hit sync button again,the tiring check database dialog pop out once more.I must select Anki 2.1 scheduler in the preferences ,If I want to use it to sync.
And 2.1.33 is the same
above all, It works repeating this operation(I go to tools-->preferences-->scheduling ,select Anki 2.1 scheduler checkbox and closes and download (not upload) in order to add/delete or change learing process)