Not working on PHP 8.x

[the-hotmann]

I successfully run this on PHP 7.4 but as soon as I switch over to PHP 8.0.5 I get this error:

AH01071: Got error 'PHP message: PHP Warning: rename(/var/www/vhosts/DOMAIN.TLD/httpdocs/SUB/archiv/tmp/K\xc3\x83\xc2\xbcndigung.pdf,/var/www/vhosts/DOMAIN.TLD/httpdocs/SUB/archiv/tmp/../../archiv/tmp/K\xc3\x83\xc2\xbcndigung.pdf): No such file or directory in /var/www/vhosts/DOMAIN.TLD/httpdocs/ph/vendor/ncjoes/office-converter/src/OfficeConverter/OfficeConverter.php on line 144', referer: https://SUB.DOMAIN.TLD/

When I run the very same code on PHP 7.4 everything works fine. So my question is:
is this problem related to my application only and Office Converter is indeed PHP 8 compatible or not?

Notice:

• "K\xc3\x83\xc2\xbcndigung" stands for "Kündigung" is this maybe related to UTF-8 encoding?

EDIT:

When I remove the "ü" from all texts and names then it works even with PHP 8, but as soon as "Umlaute" are in the filenames it breaks functionality on PHP 8