Sorry, dudes -- the problem has been in the part that I stripped off for here for I thought it wouldn't be of too big interest. I had in the fields the following additionally to the above:

fields => 'reference, subject, name, '
          . 'DATE_FORMAT(writtendate, "%Y-%m-%d %H:%i:%s AS writtendat
+e',
[download]

and as you might see I fumbled twice in this part -- once forgotten closing quote and once forgotten closing bracket... Unfortunately the error message wasn't too well so I expected the error to be someone else.

Thanks to everyone who tried to break his/her head upon this problem... This should tell me that trying to find the problem for only one hour and a half is not enough :-/
--
use signature; signature(" So long\nAlfie");