I too got the same error recently when I tried to install Emoji plugin. I had to reset all plugin as mentioned in above post and it took almost 5 restarts to get my site back online. As for why it is happening, I don't think I am the right person to answer it.
O.8.0 Bug Images with German ÄÜÖ in the file name
Images with German Ä or Ü or Ö in the file name can not be uploaded.
I see only "loading 100%"
After changing the name uploading works.
After updating to 0.8.0, I can see the same "uploading 100%", whatever the filename is.
Pretty sure its the regex in composer-default plugin that's exploding on those chars.