Looks like the index file is a bit broken for whatever reason. You could delete it (which would lose flags if you're using dbox) or you could use the attached script to run:

dovecot-index-log.py --file /path/to/broken/folder/dovecot.index.log --ext_hdr_shrink --add_ext_intro hdr-vsize --add_ext_hdr 00000000000000000000000000000000

Of course at some point we'll need to fix the code to fix it automatically..