Hi Volker,
are you running this upgrade on windows?
Did you test the connection to database with R3trans -x?
This will give a trans.log in the current directory.
What's the error there?
Did you logout after upgrading to Maxdb 7.9?
During the upgrade to 7.9 the path variables are changed to a user specific one!
So please check if the path to <drive>sapdb\clients\<SID>\pgm and <drive>sapdb\clients\bin are in PATH variable of the user <sid>adm.
For me this solved my problem, because not only dbsdbslib.dll was missing, but libSQLDBC77.dll as well.
Please stop the upgrade, login again, and resume the upgrade where it stopped. (the variables must be known in the environment ...)
Good luck!
Christian