Thanks. The main index was transferred from a 32 bits to 64 bits installation. I even changed the name of the index from defaultdb to old_data without a problem.
Just FYI. The other way around (moving 64bit buckets to a 32bit install) may cause issues if you have buckets are over a few hundred MB (700MB is the limit I think).... Normally you wouldn't want to do this, unless you had to roll back for some unforeseeable reason.