I think I found the reason why there is no text when printing: the Original XLSX came from Excel and was copied to the cloud. In Excel the standard font is Arial and Arial is not on the Nextcloud server installed. So it shows me the text on screen (but it is not Arial). If I change the font to DejaVu Sans then it works! But it doesn’t work with all font that are listed in OnlyOffice.
I will try to install some other fonts in OnlyOffice. Here is it written how to: