I was using acf free version. I purchased the pro version, installed it, I deactivated the free version, I activated the pro version and I pressed the button for the database update. All the fields that I had from the free version where there and were working well. But ALL the names of the fields appeared weird.
For example a text field with label-name: “Name of client” and name of the field: “name”, the name field didn’t appear like “name”, like it should have, but it appeared like this: