Odd php error from imp upgade

A couple of weeks ago upgraded php amd horde in my 5.6 server per Dan Brown's instructions (and excellent advice on this forum, as well).

Now I am getting some funny php errors -- nothing seems to be wrong with webmail, but I get the following when I open a mailbox:

Aug 23 17:34:30 montypython È8+^H^B[3352]: PHP Warning: ob_start(): output handler 'ob_gzhandler' cannot be used after 'URL-Rewriter' in /home/httpd/html/horde/imp/mailbox.php on line 539

...and I was wondering if there's anything I could do to fix it? It's probably some artifact of my upgrading php, imp, horde, etc. recently. The last time I had to invoke PEAR to load some updated scripts. Is this the same kind of problem?

Thanks in advance,

Ed