@ottogal, my suggestion is to delete the /site/modules/FormBuilder/ folder and everything inside it. It's not the ideal way to uninstall a module but PW will handle it okay. This should stop any fatal errors and then you'll be able to log in and do a Modules > Refresh. There will probably be some FormBuilder-related items on the "Missing" tab of the Modules section. For any item listed on this tab, click on it and then choose the "Remove this module from the database?" option.