Hi.
So I had the same issue, and as confirmed previously by @hale326 , it happenned to be an LOD problem indeed.
I duplicated and opened the project with its working version of Unreal (4.26) and deleted all the LODs I’ve configured on each assets manually to end up only with LOD 0 on them (maybe there is a command to automate the process or something instead…)
Then I was finally able to open the project in Unreal 5. Have to rework again on the assets’s LODs now =/
Hope that helps.