Deltharis was mostly right, although it took me some time to understand the exact meaning. I'll accept his answer, but want to outline what seems to have happened. I tried this both with patch 1.04 and 1.05 - this particular thing is clearly not addressed by the patch.
I loaded an older savegame right at the point when the Redanians attack Geralt by the king's order.
Problem is that at the blockade there is a Redanian soldier that is not aggressive towards Geralt until you provoke him. It's the same soldier that I couldn't kill after the cutscene in which Roche joins Geralt after the king gave the order and Geralt fought off the first enemies.
If you manage to kill that Redanian soldier before the cutscene with Roche, you simply proceed crossing the bridge. At the other end, and after killing all enemies in the way (there are none in the smaller streets), you will see another cutscene, which allows to progress further in the quest.
There is one thing that I only noticed after retrying as described above. The first time around, when I didn't get the second cutscene at the temple isle end of the bridge, there were witch hunters standing around idly along with Roche, Ves and Geralt. Those didn't stand around anymore when I got it right - they had to be fought on the way to the king.