Version 2.57 (May 20) replaced the imagemagik library with an equivalent Qt library - see
calibre - What's new - the error suggests it's something to do with that change?
But wouldn't have thought that would effect calibre server. I run 3 instances of calibre-server on a desktop Windows box with no issues.
Kovid Goyal will have to answer this, answers to these questions may be of some use.
Can you run the calibre library manager? Do you have any other tasks running that might be in conflict? Can you try terminating those tasks.
BR