Re: software updater
Double check your sources in Software Centre, Software Sources.
For the cdrom message, you might have the cd set as a source, so everytime you try to install or remove something it's looking for the cd to be inside the drive. Remove/disable any cd-rom from the sources, you would usually download what you need from the internet.
For the linux.dropbox.com error, it seems you have a source/repository set up. And it no longer exists at that address. So it is getting the 404 Not Found error. Remove this source also.
After that it should be fine. Be careful when adding repositories, make sure they work and they will keep working (not changing the URL every month).
Darko.
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Ubuntu 18.04 LTS 64bit
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