Tried to add an external form to my site and got an unexpected error message
Permalink"Unable load external form block controller file: form2.html"
I received the above error message and I cannot access the site to edit or view. Is there anyone that can help to resolve this problem?

http://www.concrete5.org/community/forums/chat/external_forms_file_...
...which describes a process that seems to work for using an external form. However, as the OP did, my initial stab at an external form was creating an HTML file which means I'm at a loss as to how exactly I might create a PHP controller for it. Hmmm....
Go into your Dashboard > Full Sitemap
Click on the page where you tried to add the external form and select Versions from the menu that pops up.
Select a previous version of the page, before you tried to add the external form, and click the Copy icon in the top right corner of the window.
Check the new copy (should be at the top of the list) and click the Approve icon in the top right corner of the window.
You just rolled back your page so there's no external form. Hooray! Now you can access it again. Hooray!
Now, here's a thread I found with the procedure for adding an external form so that things don't break. I just tried it and it works! Hooray x3!
http://www.concrete5.org/community/forums/chat/external_forms_file_...
One more note: It appears to take some time for the database to recognize changes to your external forms and their respective controllers, so take it slow and don't panic if you get a database error. Just walk away for a couple minutes, come back, and hit refresh.
Hope this helps.