I just installed XOOPS, and everything was working fine.

I set up extCal, which seemed to work fine.

Once I added an "Upcoming Events" block to the site, I started getting this:

Warning [Xoops]: Smarty error: unable to read resource: "db:system_block_mainmenu.html" in file class/smarty/Smarty.class.php line 1084

I did a bunch of searching on the forums and Google. Everything seems to suggest making sure write permissions are set on templates_c, uploads, and cache:

heavy htdocs # ls -ld cache templates_c/ uploads/
drwxrwxrwx 2 apache apache 4096 Nov 1 15:13 cache
drwxrwxrwx 2 apache apache 4096 Nov 1 15:08 templates_c/
drwxrwxrwx 2 apache apache 4096 Oct 31 21:33 uploads/

Any help?

Try updating the system module through System Admin > Modules and re-generate the templates if you cloned them. That *should* fix the problem.

HTH