Indeed the -a switch apparently translates to -rlptgoD where p is for perms isn’t it? 
Will try other switch combination for rsync and report back here. Besides - as a dirty workaround - if I am not able to fix otherwise, I will restore to default access rights as described by dnutan ( signal-event ibay-reset-permissions sharedfoldername) everytime a rsync is done.
I’ll test and report back here.
ok, by try and error I found that the following switch does what I need; copy over the files while preserving the access rights on the destination drive thus remaining read-only on the destination drive:
rsync -rlt -e ‘ssh -p portnumber’ root@sourceserver:/var/lib/nethserver/ibay/sourceshare/folder1 /var/lib/nethserver/ibay/destinationshare/folder1
thanks to everyone posted and helped in this thread