Logged In: YES 
user_id=1170021

I have no problems any more, because the issue only arose
during my initial conversion of the old database. I managed
to get it converted by hacking Database.serialise() in
back_anydbm.py. I used something like,

        elif isinstance(prop, Date) and v is not None:
            if type(v) == tuple:          # inserted
                d[k] = str(v)               # inserted
            else:                             # inserted
                d[k] = v.serialise()

and then I could convert the database successfully. Then I
removed the hack, and everything has been fine ever since.

So I am not concerned about this particular issue any more,
but maybe somebody else will be some day!