To address this longstanding UE issue with corrupted structures open affected BPs (see Output log) and select a function with a structure and right click refresh. For affected interfaces briefly change the structure to something else, then switch back. However, as mentioned if you already upgraded to 5.6 from 5.5 it may turn out that the Output log no longer displays affected BPs. For the time being use 5.5 if you can, otherwise take a look into the logs or check BPs in your project which may be affected and refresh those structures.
Also if you manage to fix these structures, you then could try to upgrade to 5.6, but caution is warranted.
ping @Kikilo