We're fundamentally rewriting our filtering and search code to meet the needs of the archive at its current scale. It's a big undertaking (we've been referring to it internally as Project Mordor), and it's required a lot of coding and testing, but we hope to have the new system out to you as soon as humanly possible. The AO3 team is full of archive users, and we're eager to have the searching and filtering options available once more. Thank you for your patience!