Have you tried running the process by just that assembly?
or just the phantom assembly?
I haven’t received that exact error but it seems like I’m getting more errors now in E10 than used to in E9. Those errors don’t have many specifics either (at least not that I can easily understand).
Like you, I look in the log for the last part number that tried to process Mfg Lead Time - and eventually will find the source of the error somewhere in it’s methods or related planning settings.
I find it it has helped to run the process for the specific part(s) that were failing.