Trouble saving files after i have made comments/highlighted some text

I have used this program (Adobe Acrobat Reader) for months for taking notes for my study, but suddenly i get this message when i try to save a document, and it just gets blank.

The error message is (roughly translated to english):
"The document couldn't be filed (or archived). The file may be read-only, or another user may be accessing it. Save the document under another file name or in another folder". It doesn't appear read-only though, as i can edit it for a certain amount of time before the problem arises.

If i then try to do one of the options, it says:
"The document couldnt be filed (or archived). Wrong parameter."

Have any of you tried this before? how do you fix it?

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