Hello Richard.
Thank you for the answer.
We were working on that and yes, we discovered that some files (xml) were pointing to nothing at all.
(All the files that have given us the problem before were on the same Event so we narrowed that quickly.)
It seems that looking for those files and deleting them in the Asset folders and the depot (plus deleting some cache files aswell) has worked and we don’t have the error anymore.
Thank you 