Hey there @CedricO and @ehdrbfb ! Welcome back to the community! This is gonna be a fun one. So let’s start with what we know and work into an assumption.
The error Assertion failed: Header.Len != 0 is failing because it expected the Header that got passed in not to be 0. Since we know Fname is part of an Object’s data, we can assume that it could be literal in that an object is missing a name when queried, or is corrupted. This can also occur if your file paths are far too long (in total), though that has a different error usually attached.
Have you deleted/moved anything from the project outside of the editor?
How long is your largest named asset’s file path?
Are there any completely blank named assets that were imported? (this could be impossible to see from outside of the engine so don’t sweat it if you can’t tell.)
Was this project migrated or built in this version?