[exim] skim with yahoo fails ...

hi there

i've a problem with dkim and yahoo.

the log shows me something, it seems very special
-----------------------------------------------------------------
2014-09-23 12:00:06 1XWMtC-000Dzw-0C DKIM: d=yahoo.com s=s2048 c=relaxed/relaxed a=rsa-sha256 t=1411466404 [verification failed - signature did not verify (headers probably modified in transit)]
2014-09-23 12:00:06 1XWMtC-000Dzw-0C DKIM: d=yahoo.com s=s1024 c=relaxed/relaxed a=rsa-sha256 t=1411466404 [verification succeeded]
2014-09-23 12:00:06 1XWMtC-000Dzw-0C DKIM TEST: domain=yahoo.com possible_signer=yahoo.com status=fail reason=signature_incorrect
-----------------------------------------------------------------

as you can see there are two lines. one succeeded and the first fails.
the difference seems to be the s=1024 vs s=2048.

i could not find any documentation on this thing :)

any help is very welcome.

kind regards
Karl-Heinz