I installed cpu-x via the “Software hinzufügen” (=add software) program, there are no further updates visible
The Discover program, strangely enough, reports two available 0 byte updates as seen on the screen shot and refuses to install them with the unknownError message
Any issues with installation of software from the AUR would best be reported to the maintainer. I would suggest not mentioning Manjaro as AUR is (ARCH user repository).
Understood, I just deinstalled Discover (although I find it a bit awkward that it shows up in the official repositories while it is not recommended).
Case closed for me