Solved.

Webroot SecureAnywhere's identity protection service denies copy and pasting from applications it thinks may have sensitive data -- like MyInfo. Instructions for enabling copy and pasting can be found at (in case anybody else asks):

http://www5.nohold.net/Webroot/Loginr.aspx?pid=10&login=1 ...

It is not a very original suggestion but as I have no better idea and there are no other advices:

You could uninstall and reinstall MyInfo in order to see if it works then.

And something else:
In case that you use a special clipboard manager for copying/pasting: deactivate it temporarily and test ...

I’ve just purchased MyInfo Standard, and am encountering difficulty pasting content into documents. Specifically, I’m unable to paste:

URLs copied from Firefox’s address bar
Text copied from a web page in Firefox
Text copied from a text editor

I am unable to paste regardless of whether I:

Press ...