Same problem, CC2018. It only happened after I exported it once, password-protected the PDF using Acrobat. Now the PDF won't let me take the password protect off and I'm having this problem trying to export the InDesign file.
You're not giving us enough information to help you. You're describing two issues: You exported the PDF from InDesign once, but it won't export again? And, separately, you password-protected the PDF using Acrobat, you can't turn it off?
What operating system? Did anything happen in InDesign between when you exported it successfully, and when you tried to export it again? Have you tried to restore your InDesign preferences?
Regarding the password protection, you'll have to ask about that in the Acrobat forums.
Yes, I exported the PDF from InDesign once, but it won't export again.*
Yes, I password-protected the PDF using Acrobat and now can't turn it off.
Operating system - Mac Pro OS X El Capitan v 10.11.16
*Your suggestion to restore my InDesign preferences resolved the InDesign export to PDF issue.