You are running a custom feature there named MegaGallery which I suspect implements search but has not been updated to be compatible with recent changes in our search index.
I would look for a file corresponding to mega gallery in the folder /Setup/ProviderConfig/indexbuilders
remove the file, then touch your web.config file and it should prevent the error. Then contact the developer and ask them to update it to be compatible. I had posted in our developer forum so that developers would know about this change and be able to update their custom features.
Hope that helps,