You don't say which version/release of Zimbra you're using but here's a couple of thing to try. First do a 'zmprov ga firstname.lastname@example.org' and have a look at the Sieve fields, make a copy of the data in there. Next look for the field
and it should read as follows:
It's not quite clear which filter you're actually talking about but the Initial Search String is the first one to look at, if that's not set to the default or has an invalid entry then a user login can crash. Reset the field to "in:inbox or is:unread" and try a login to see if that's the problem. If it's the Sieve fields then I'd suggest you save a copy of the filters and reset them to a single valid filter with the following command:
zimbraPrefMailInitialSearch: in:inbox or is:unread
If that works you can add the original filters (less the problem one) again later.
zmprov modifyAccount email@example.com zimbraMailSieveScript 'require ["fileinto", "reject", "tag", "flag"]