"Most likely you ran out of space for rules. There is a limit in Exchange to how much space the rules can take per mail account.
Quote from an answer here:
There is a size limit for the rules, not a an amount limit. The limit is in Exchange and not in Outlook. Exchange 2003 and previous have a limit of 32KB. Exchange 2007 has a default limit of 64KB but this can be extended to 256KB by your mail admin.
More info and suggestion about what to do here and here."