A user's migration is incomplete and the error "Missing session string , Code: ea10, info: null" appears in the log.
It was found that the user had 4800+ messages in the inbox. The SOAP protocol appears to have issues with large folders ~5000 message.
Moving the messages into folders, in this case sorted by year, solved the issue.
This article was originally published in the GWAVA knowledgebase as article ID 2400.