>Set BackStyle to Transparent in Format menu in report designer.

This was one of the first things I looked at, but all labels and textboxes were already set to transparent from the beginning.
My guess was that there is an issue with the way how the print job gets generated using Reportbehavior=90 in a citrix environment, and having different versions of fonts installed on the citrix client and the print server.
But why it did not happen with landscape reports is hard to explain.