I think the s3 server sync for your location is not done yet. So wait a couple of hours and try refreshing and updating again.
I refreshed Update Manager again this morning [= several hours after your post]. The only new update it found was chromium.
But I guess the other packages could be redownloaded and installed successfully.
I took the update ! Working fine here ! Thankssssss !!
I do not understand that what you're writing about Neon or Maui 2 linux. Plasma 5.8.3 does not update in Maui 2 linux.
KDE Neon now has P5.8.4
I was writing about both KDE Neon AND Maui. What appears in Neon re Plasma appears a couple of weeks later in Maui (if you enable the Neon backport in Update Manager], once the Maui Devs have tested the new Plasma version for stability & compatibility. Yes i know that Neon has recently gone to P5.8.4, & we Maui users will presumably get it too in a week or two. There are various mentions of the Maui Neon backport earlier in this thread, & in other threads.