Error appearing in logs every time settings are changed

I'm new to drupal, so I'm not sure whether this is a bug in the code, or something I've done wrong with my configuration, but I thought it best to put it up here, just in case.

Every time I change my settings on the module's config page, this error appears in my logs. As far as I can tell, it does not cause any problems in the module's actual operation, though.

Invalid argument supplied for foreach() in /home/fraz/public_html/o/modules/reptag/reptag.module on line 708

At present (this is a non-public site, testing things before it's ready to go live), only authenticated users have permission to use reptags, and they are only allowed to use one tag module - the bbcode one, with a few tags stripped out of the code (I did this myself, manually by making a copy of the tag module and editing it). I'm reasonably confident that this is not the issue, though, as the error persists even when the tag module is disabled.

I don't know if this would make any difference, but I do not have xajax installed.

Also, I'm not sure if this warrants a separate bug report, as I can't verify where it originates, but when I checked the "Enable Debugging" box and saved settings, I received a blank white page with only the word "NULL" printed at the top. Every subsequent time (I tried at least 3 times) that I went to the page to turn this option back off, I received this, and found that the option had not been disabled. I opened up the database to search for the appropriate flag, to switch it off, but before I had touched anything, I tried to change the option one last time... but it worked this time.

The error that appeared in the logs with this odd behaviour is this:
Cannot modify header information - headers already sent by (output started at /home/fraz/public_html/o/modules/reptag/reptag.module:254) in /home/fraz/public_html/o/includes/common.inc on line 266.