I'm sorry about the troubles you are experiencing when trying to edit text in your block. When you are making these edits, are you copying and pasting the text from another source? I would recommend to try clearing your styles on the text in your block, or pasting your text into a text editor like Notepad first, before adding the text into the Constant Contact editor. This will strip out any addidional coding that could be causing the parcing error you are receiving.
Is the content you are editing any kind of symbol like Copyright, super/ subscript, or Trade Mark? If so, those would need to be added in the HTML coding section of your email. You can always try going into Private/ Incognito mode as well or try using a different browser like Chrome or Firefox.