We need a bit more information.
Does this happen with all PDF forms, or just one specific one?
I've never seen a problem with this function, which means we are not talking about something that is always failing, but may have to do with the specific field, or with the form you are editing. Would you be able to share your document?
If it does not happen with all documents, or with all fields, I would try to re-create the field in question and then see if the new instance of that field is behaving correctly.
So basically, the problem is happening with all my pdf files, but only on my computer. I have tried to make the changes (limit the number of characters) with the same files and on different computers (on windows and on Mac) and it worked without any issue.
Sorry I am not allowed to share these documents, I'm working in an Insurance company with really strict security rules.
What I noticed is that my problem appeared when I installed the Adobe CC softwares, I was using CS6 softwares before that.
I would try with a brand new document: On a Mac you can create a blank document: Make sure that no document is open (e.g. just restart Acrobat or close all open documents), then use Cmd-Shift-T to create a blank document. Now add a form field to that document and apply the same changes you've used for your real form. Does it also show the error?
So I followed your instructions step by step and I still have the same problem. And I also deinstalled Adobe Creative Cloud and reinstalled it again and it didn't fix the error.