If you create a lead and assign it a planned call and, after doing this, you convert the lead to contact you are unable to access activities panel any more. When you try to do it you receive an eror like: "Error filling in additional detail fields: Query Failed:SELECT name from contacts where id = '39b82c5a-7da6-2be0-987f-42f0b5a565ff'::MySQL error 1054: Unknown column 'name' in 'field list'"
The converted call (I think) seem to refer to contacts.name field which does not exist. To solve the problem you need to manually delete the call in MySQL calls table.

Hope this can help someone.

Luigi