Try reuploading all the CMS Builder files to see if that helps. 

If that doesn't help, send in a Support Request here (don't post details in forum):
https://www.interactivetools.com/support/email_support_form.php?priority=free&order=NA&message=forum-posts.php?Error-with-Install-80771

In the Summary of Problem include the CMS Builder Product ID to use and MySQL details (hostname, database, username, password).

Thanks!

Thanks for this - I'm really not sure what that all means though?! Which kind of plugins could it be and how would I find the code for these to check?

Hannah

Dave wrote about this issue and the solution here:
http://www.interactivetools.com/forum/forum-posts.php?postNum=2239457#post2239457

What it means is that a plugin was updated and it's got some extra whitespace at the beginning or end.  Does this error happen consistently?  If so, try turning on various plugins and reloading.  Then check the plugins (or all plugins for whitespace after the close ?> tag.

Try those steps and let us know if that resolves the error.

Thanks!

May not be the cause of your issue, but I had this exact situation present itself due to the opcache issue discussed with Dave here:
https://www.interactivetools.com/forum/forum-posts.php?postNum=2239627#post2239627

Again - I'm not saying it's definitely the cause of your issue, but it could possibly be if you're hosting with Rackspace or another cloud provider that disabled said function. The opcache error had output which triggered that install error, effectively halting the install process.

Steve

Trying to run an install and getting this error message:

  Unexpected output was sent by a program library or plugin.
Output was "
" (2 chars)
Developers: Check plugins and library files for accidentally added whitespace or other characters.

Any ideas how I can fix this? Thanks!

Hannah