I have had several issues with external references in Solidworks 2019. When I examine the external references, I see a red x next to Base flange (see image below)

I deleted all external references in that part, then re-inserted but the red x won't go away.

What puzzles me is that the relation is marked as in context, and all the information is present.

What does the red x mean?

I use a plug in called Customtools a lot, and customtools is currently causing solidworks to crash. Customtools developers are blaming this strange x in the external references. They have helped me try to find a work around, but warned me that this is going to cause issues.

I have seen this in several assemblies, with all parts created in 2019, both my own work and my colleagues work. I have attached a copy of the assembly.

This is causing a lot of problems for us, anyone know if there is a way to fix it? Or is it a bug?

FYI I'm not on maintenance, so can't send it to VAR. We are using SW 2019 SP0.