Hi,
thank you for your reply!
Yes, Version 5.0.14 is installed. It is a multisite installation.
We use neither blocks nor shortcodes but the Transform Post Enclosure feature from the settings′ page which works fine with other blogs. It is this blog in particular where the error occurs. It happened on another blog as well but resolved itself.
Weird thing is that one file has a wrong file extension and there the player works. It is a MP4 container (with no video) with file extension .m4a.