I had my client sign a contract and return it to me for co-sign, but it will not allow me to sign.

I have the same issue. I made a licensing form in InDesign, put it in Acrobat DC, added fields for 2 co-signers, locked them and made them required, applied security to it (ok to print & enter fields only). The first user used HelloSign and returned the document to me (they saved it and renamed it) but then I couldn't even click in any of the empty co-sign fields that I need the second person to sign - they were just gone. I had to export it out as a Word doc, save as PDF and reapply the fields I needed for the second co-signer. I don't see anything on HelloSign's support site for this. Did I need to remove the requirement off my fields & keep them on theirs, or does HelloSign remove everything else once it's signed through their site? (Yes I realize this is Adobe not HelloSign, just wondering if anyone else has experienced this as I'm not familiar with these other signature signing sites. I supply the user instructions that my form works best in Acrobat, but I need to be aware of what happens if they don't use Acrobat, or warn them not to use certain ones.)

How did they sign it, exactly? Did you add a digital signature field to the file? If so, did you set it to lock other fields?