I would suggest using an application called Disk Usage Analyzer available from the AppCenter to determine where the culprit is. With that information you can determine what needs to be removed and figure out the best course of action. The best solution will depend on whether the files are system files or user files in your home directory. The aforementioned ...
I haven't done any setting, but after last update today the issue disappeared.
Therefore, now copying a file into Files and pasting into terminal/text editor does the right this, e.g. it pastes
I wonder where this has been fixed.