Migrating from local Exchange to Office 365/exchange online

thank you for your fast answer, Vasil

"What did you specify for the TargetRootFolder in the mapping file? "
nothing.
in the meantime i try "\", it looks like it solve the problem

"why are you using the PST import method if you have on-premises Exchange? "
i read the documentation for migrating and there was the hint, that the cutover migration is for < 2000 mailboxes
and the hybrid for > 150

i have just 8 mailboxes and 3-4 shared mailboxes

i will try the hybrid version, buit i am not shure, if i can setup the local exchange server for directory synchronisation. maybe there are some "not so good setups" from the last sysadmin

ps:
i am since 2 weeks on my new workingplace after some month of "out of IT business"
my last exchange server, which i handled, was 2005 ;-)