Hello Lorraine @herbsandhelpers,
I'm very sorry, I can't find it again. I'll try to approach the problem as far as I remember.
The error is produced if html output is done before the correct html declaration is output what should be for 2.3. stores in template_top.php:
older 2.3 versions:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR...nsitional.dtd">
Any other php file which is loaded in application_top or the page file before template_top, should not output any html code.
So a blank line or even whitespace at the end of any php file, after the last closing ?>, may produce a html output.
Now for your concrete error:
If it is only produced when you switch SEO URLs to rewrite, the fault should be in the SEO URL file which is used for rewrite mode instead of standard mode.
Ergo I would check the following file:
If this doesn't help, check all other files in the ultimate_seo_urls5 directory.
If this doesn't help, search this forum for older posts related to this error.
Older versions of the iosc mobile add-on had several files included with blank lines and produced that error.
If you neither find the solution there, try a google search for more generic solutions for that error.