Trac API datetime Error when creating new wiki pages

Question

I'm trying to use XML-RPC for creating new Trac-Wiki pages. My code:

from xmlrpc import ServerProxy
proxy=ServerProxy(getPageURL(),context=getSSLContext())
params = {'sum':"Page newly created",'minor': False}
proxy.wiki.putPage("newwikipage","hello wiki!",params)

yields the Error message

xmlrpc.client.Fault: <Fault 1: "'unsupported operand type(s) for -: 'str' and 'datetime.datetime'' while executing 'wiki.putPage()'">

What I've tried:

• passing a datetime.datetime object instad of "newwikipage" throws another error ('datetime' object has no attribute 'split')
• use_builtin_types=True and changing params didn't help
• manually creating wiki-pages works
• all other API functions I tested work, including ticket and component creation (so it's probably not a faulty SSL context)

Any other ideas?