I have save files contained Swedish characters, LO won't open them again

I have files with names containing å, ä. ö. Now when I try top open any of them LO says the file doesn’t exist. If I rename a file without these UTF characters I can open it. This behaviour appears to fluctuate but I do no understand why.
Running Debian Buster and LO

I have lots of files containing non-standard characters, among them the German “ä”, “ö”. These should anyway be the same Unicode code points as used for Swedish.
Thus I cannot imagine that your issue should be related to special charactrs, except you used a very uncommon way to insert them, probably. How did you do it? Why do you emphasize “UTF-16”? The UTF encoding should only be set explicitly for import of plain text files into Calc sheets.

And what’s your locale?

Is the problem that the Filename contains the Latin-1 characters, not that the file itself contains the characters. I assume that you are saving the files with the normal .odt format.

What do you mean by UTF-16. LibreOffice Unicode uses UTF-8. Where did you define UTF-16?

OK,agree: The names contained special characters. I hadn’t realised that when writing my first comment.
The handling of file names is a matter the OS should be charged with, and I cannot test with any linux.
Then: Why? I never use filenames containing special characters or spaces, and my files feel well with their names. However, if I try a name containing the mentioned characters under Win 10, I don’t experience any problems.

I have no problem with Linux-Mint, Language Eng-UK in creating a file containing Teståäö.odt and saving it with that filename. Perhaps the problem is with UTF-16. But we have had no reply as to how and why you are using UTF-16.

(What proceeding is needed to force an OS to use a specific character encoding in file names?)

Most Linux distros use UTF-8 by default nowadays, but you can change it by changing the locale. No idea about other OSs.