@humorkritik is referring to this post.
Apparently @Horst_Schlammer solved the issue by directly installing the Storage Upgrade for 1.8.5. It should take you directly to Fairphone OS 1.8.5. Go to the Fairphone Updater > Advanced Mode > Android for Fairphone > Storage Upgrader. (The headline might say “Older versions” (“Altered Versionen”), but that is an unfortunate naming.)