This bug in OpenOffice / LibreOffice has been with me for years and years. You open a file, you delete it while open, you close LibreOffice — and then LibreOffice will henceforth tell you eeeevery time you start the program that it tried and failed to recover that file. But I found a bug fix. Thank you “user177723”!
1. Open LibreOffice and create a new file with the same name as the lost one in the same directory. (Which directory this may have been, you have to remember/guess.)
2. Save it.
3. Close LibreOffice completely. (Yes, this is an essential step.)
4. Open the newly created file. Close it.
5. Repeat for every individual file the stupid program moans about on start-up.