I know the title is absurdly vague, but so far it's really the only thing I can think of regarding my issue. I'll copy/paste what I've posted at the TT Marketpress Frontend Admin forums, as this is related to the plugin:
"I’ve identified an unusual problem, don’t know if it’s happening only with me: the plugin is returning me an error whenever I deactivate it and reactivate it:
“Warning: Cannot modify header information – headers already sent by (output started at /home/wheelofcommerce/public_html/wp-admin/includes/template.php:1877) in /home/wheelofcommerce/public_html/wp-includes/pluggable.php on line 1178?
This is what’s in line 1877 of template.php:
<html xmlns="http://www.w3.org/1999/xhtml" class="ie8 ” <?php
And this is what's at line 1178 of pluggable.php:
header("Location: $location", true, $status);
I've tried deleting the pages it created, deleting the plugin itself and reinstalling it all over again, but still got the same error. Same thing happens with Twenty Fifteen as theme and with all the other plugins deactivated, so it's pretty obvious it's something in TT Marketpress Frontend Admin that's causing this.
BTW, I’ve successfully applied all the changes from that “Create an eBay-like site” tutorial, everything was working perfectly. Besides, like I’ve said above, the error persists even after switching the theme to Twenty Fifteen and deactivating all plugins except for TT Marketpress Frontend, and as all modifications made following the tuturial are applied to the child theme and by installing plugins, the changes made by the content of the tutorial simply can’t be the cause here.
Any idea on how to fix it?"
And it's not only that: as I was trying to get the plugin to work, I've deleted it and reinstalled it several times, each one of them doing something different, and now, when I log into one of my test accounts and go to the store admin area, everything seems fine, except that the slug is /store-admin-6, but I don't see 6 store admin pages listed at my wp super-admin dashboard (if I remove the -6 and press enter, it redirects me back to a permalink with the -6 in the slug). I also don't see the other pages create by TT Marketpress Frontend Admin listed anymore.
Considering all of that, I think I might be facing a caching issue here, even though I haven't used any caching plugins for a while now -I have WP Super Cache installed, but it has been deactivated for some time. Or it can be something else that's causing it, I honestly don't have a clear idea on this.