we have two SLP DAs (OES 11 SP2). Both are listed in Novell Client
configuration; if both are up everything works fine, the same, if only the
first is up.

If the first DA is down and the second up...

.... some clients successfully log in after a delay of about 30 seconds -
this is the behaviour I'd expect.

.... but most clients after that delay say "tree not found"; if we try again
login is successfull.

I think that scenario should work, shouldn't it? If one DA is down, Novell
Client should be happy to use a second one?

slpinfo /a shows correct information - again after a delay when it trys to
access the first DA.