Just realised (as of last night) that the search form is actually broken. Who knows, maybe has not been working since I upgraded to v2.63? Will try to resolve it during this weekend. There should not be any problem with the underlying MySQL database, since in editing mode, search is possible.
yy
Well, after reinstalling MT (courtesy of shifting to the new web host), the broken search form is fixed! Go figures – I will never know what got broken in the first place…