Question: When i click on the buy button in the software center it instructs me to login, however as i'm using Keepass for saving my passwords i would like to paste it in but it wont let me...

the same applies to gwibber when i try to input my facebook password.

it is possible to input manually but since i picked a long password this would take very long.

Unfortunately until now i couldn't find a solution other than just not logging in or stop using the password manager.
Does anyone have an idea?

Answer: 
I solved this problem by using keepassx instead of keepass.
I also experience the scenario in Gwibber where I copy the username/password from Keepass but cannot paste into login fields using the mouse paste menu or ctrl + v. The workaround to this is to

* copy the login detail (user name or password) from within Keepass per usual
* switch to a text editor such as gedit
* check if the mouse paste menu or ctrl + v works there (on my system they do)
* highlight the pasted text in the text editor and copy (I use ctrl + c)
* switch to Gwibber's login screens and now paste the text using ctrl + v
* the copied text will now appear

I have applied this approach to several applications where my details would not paste directly out of Keepass. (including - if I recall correctly - the Ubuntu Software Center)