Thanks mouser, in synch mode, I couldn't find the option you mention.
However, I think I found the problem.
All my folders in the source have a very recent date, which is wrong.
Some evil program may have changed the date in folders (not in files).
BC is fooled and thinks all contents must be different. (scratch that, not all folders).
But something really funny is going on with folder dates. they don't match.
On top of that, plety of files that have the exact same date are still detected as different (AND they are not of course) by BC.
If I find the culprit, I'll make sure everyone I know hears about this.