As the message says, there may be a problem with the imports.
Try to put a print('importing module name') at the beginning of every module and see what’s importing what.
EDIT
I also have noticed that the server modules are imported in alphabetical order, and that may cause some nasty surprises: In what order are server modules imported? - #2 by stefano.menci
Or I may be wrong, the modules are not loaded in alphabetical order, or they are loaded in alphabetical order only in certain conditions, and this could explain why you have hard time replicating the problem.