Re
 

That's the weird thing - I changed the theme to the main one which has a tpl file for sure, aand this is still popping up.

Maybe your theme settings is wrong? What is the default theme in your table prefix.settings? And what is the theme used by the admin if you read from the table prefix.users? Note a typo can completely messes up your site, and the theme name is case-sensitive.

Re
 

The default theme in settings is "main", but I can't seem to find anything theme related in prefix_users

Oh sorry, you need to look up the table prefix.users_options to see what the theme settings is for each user.

Re
 

Thank you! That fixed it somewhat! *feels dumb* Now I just need to figure out what went wrong with the other theme...
Edit: do the template files have to have certain permissions? I have a file backup of the the theme when it worked, but it's still giving me errors...

Fatal error: Uncaught exception 'SmartyException' with message 'Unable to load template file 'template.tpl'' in /home/wallie12/public_html/inc/smarty/sysplugins/smarty_internal_templatebase.php:127 Stack trace: #0 /home/wallie12/public_html/inc/smarty/sysplugins/smarty_internal_templatebase.php(374): Smarty_Internal_TemplateBase->fetch('template.tpl', NULL, NULL, NULL, true) #1 /home/wallie12/public_html/classes/class_frame.php(226): Smarty_Internal_TemplateBase->display('template.tpl') #2 /home/wallie12/public_html/classes/class_mysidia.php(449): Frame->render() #3 /home/wallie12/public_html/index.php(86): Mysidia->output() #4 /home/wallie12/public_html/index.php(90): IndexController::main() #5 {main} thrown in /home/wallie12/public_html/inc/smarty/sysplugins/smarty_internal_templatebase.php on line 127

O.o It's back again... and now, I have nothing to compile to fix it.

Did you make some changes to the theme and template files this time?

Re
 

Not to the template file. Theme, yes.
Edit: Okay... so it works when I'm logged on but not if I'm a guest? O.o