I met same problem and it was resolved.
You can use the Action Wizard: Tools > Open Action Wizard > New Action.
Choose tools to add: Move Protection > Encrypt and Save and Export> Saveto the right-hand panel.
Choose Encrypt > Uncheck prompt user box > Click “Specify Settings” and select “Password Security” in Document Security dialogue box > Check box “Require a password > Input password.
In this step, I will save encrypted pdf files to a new name. Choose Save > Click “Specify Settings” > In Output Options Box, 1. Select “Add to original files name”, Chose to add text BEFORE or AFTER the original file name. 2 Save File(s) As Adobe PDF > OK and name the action.
Click this new action > Click "Add Files" to batch select files > Start > done
I hope this helps.