Avatars messed up after updating to recent Confluence version

After updating Confluence from 4.3.7 to 5.4.4 the avatars are messed up. Some user images are not displayed and where they are shown it seems that these are images that were used before.
What we've checked is that all new images are available in the updated environment. That means that somehow the references to the new images have changed.

What can we do to get the reference corrected?

Thanks in advance,

Thomas