I had same problem on macOS with GraphicsMagick. Try to switch GraphicsMagick to ImageMagick in app
default-image-converter : im
Then don't forgot to restart app.
CMS has a small protection for testing credentials. If some user performs more than 5 attempts to login then the system blocks IP for 15 minutes. So maybe is your IP blocked.
Uploading files is limited according to rules below and you can extend this size in: