Faq/General/Defender Controlled folder access exception
Setting up Defender Controlled folder access exception for LibreOffice
The Defender Controlled folder access feature might prevent LibreOffice to save files to user's folders. In this case, "General input/output error while accessing Document" or "File not found" errors will be shown.
To workaround this, add LibreOffice main module (C:\Program Files\LibreOffice\program\soffice.bin) to Defender Controlled folder access exclusions. Please refer to this Microsoft knowledge base article for details. Note that initially, soffice.bin will not be shown in file chooser dialog, because only COM and EXE programs are shown in the list. You may need to enter *.* to the File Name box and press Enter to make the dialog display all files, then select soffice.bin; or just type the name difectly when in C:\Program Files\LibreOffice\program.