So I have solved this issue, but now I have another problem:

An unresolvable problem occurred while calculating the upgrade.

 This can be caused by:
 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu
 * Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

I am not upgrading to a pre-release version of Ubuntu and I am not running a pre-release either. I have unchecked all my 3rd-party packages using Ubuntu Software Manager, Edit>Software Sources...

What else might be wrong?

UPDATE

After doing sudo update-manager -d and sudo apt-get update;sudo apt-get dist-upgrade as per JimB's post, and then running sudo do-release-upgrade, here what I get:

Err http://extras.ubuntu.com trusty/main Translation-en                                                                                              

Err http://extras.ubuntu.com trusty/main Translation-en_US                                                                                           

Err http://extras.ubuntu.com trusty/main Translation-en                                                                                              

Ign http://extras.ubuntu.com trusty/main Translation-en_US                                                                                           
Ign http://extras.ubuntu.com trusty/main Translation-en                                                                                              
Fetched 0 B in 0s (0 B/s)                                                                                                                            

Checking package manager
Reading package lists... Done    
Building dependency tree          
Reading state information... Done
Building data structures... Done 

Calculating the changes

Calculating the changes

Could not calculate the upgrade 

An unresolvable problem occurred while calculating the upgrade. 

This can be caused by: 
* Upgrading to a pre-release version of Ubuntu 
* Running the current pre-release version of Ubuntu 
* Unofficial software packages not provided by Ubuntu 

If none of this applies, then please report this bug using the 
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. 


Restoring original system state

Aborting
Reading package lists... Done    
Building dependency tree          
Reading state information... Done
Building data structures... Done 
=== Command detached from window (Mon Aug 18 23:53:10 2014) ===
=== Command terminated with exit status 1 (Mon Aug 18 23:53:10 2014) ===