Are you a CB Documentation Subscriber? They tend to give highest support priority to those.
I have a feeling there's something in your Community Builder configuration that's causing the error. The Username field is required and populated for all your users, yes? Also if you have "Allow change of username" enabled (not recommended : use Display Name field instead) it can break many things after someone renames their username (mainly cache but some poorly-coded extensions can break)
Make sure you're on CB 1.2.1 - 1.2.0 has a huge security hole and is not supported anymore.
For some reason, plugins/content/cbfieldload.php doesn't exist on my installation.
What version of Community Builder do you have installed? If it's not 1.2.1 please update, I don't think you'll get much Free Support otherwise. If it is 1.2.1 already, then it's a problem with a 3rd party plugin that I have never seen before. Or you're using Joomla 1.0 or something....?
....don't know if this is due to using cb profile pro
What is "CB Profile Pro"? EDIT: I just checked it out. I guess its the plugin that's causing the problem, try disabling it and testing it.
If it was free, I would download CB Profile Pro and bugtest. But it's not free. And i'm not paying 13 Euro for a software that seems to do nothing except change CSS and layout
Maybe we can contact them and try to arrange something, but I don't quite understand what this extension does yet