Unable to Combine Modification Applications into One PDF

I'm trying to combine several modification applications into a single PDF. However, when I use the Combine or Organize tools to add the files, I get the following error message: 

I’ve been able to combine PDFs without any issues in the past. Has something changed recently? Is there a workaround or updated method I should be using?

TIA for your help!

Jenna

P.S. Some of the Modification Applications are fillable forms - they all contain the same form fields.