Any Webmaster and Anonymous User can access all parts of my site fine, but registered users get this message when trying to call up any page:

Warning: filemtime(): Stat failed for /theme.html (errno=2 - No such file or directory) in i:\ptownvoice\class\smarty\Smarty.class.php on line 1650

I checked group permissions, so they couldn't have been accidentally reset.

My smarty.class.php file has the following on line 1650:

$template_timestamp = filemtime($resource_name);

Any ideas as to how I would go about fixing this? Any and all information is greatly appreciated!

Make sure all files in the themes directory are uploaded. Try uploading themes that you have removed recently, if any.
If these still don't solve your problem, try the following fix:
http://cvs.sourceforge.net/viewcvs.py/xoops/xoops2/include/common.php?r1=1.38&r2=1.39

Thanks for the reply, Onokazu!

Before you replied, I tried to go in as a webmaster and change the theme to the default XOOPS one and then switch it back; that seemed to rectify all the problems.

I'll be sure to use that fix if it happens again!