Mods tend to cause problems ... when I see mods like spam protection, pretty urls, tabs and custom pages, it starts ringing bells.
Or maybe file permissions set incorrectly. You can check the correct file permissions using ftp client and the ones given in Simple Portal install documentation.
I would check all the file permission setting first before anything else. If that does not work then move on to examine other mods.
Also any mods you might have uninstalled may not have completed removed code - often mods that affect access and urls change the htaccess file and do not reverse changes after uninstalling - worth checking.
The only way to see if it is a mod causing trouble is to uninstall the others. You do not need custom pages if using simple portal - it has its own page system. So I would copy your pages over to the Simple Portal system and uninstall the custom pages mod.
If I were you I would uninstall all of them first and test the site with just Simple Portal. Try and repeat the error.
Add mods back in one by one and try it again.
I have aeva media and highslide installed, and I cant see the smilies pack being a problem.
Files permissions probably then consider Stop Spammer , Custom Pages and Tabs, Custom buttons / tabs with Sub Menus.
And one final thing - what is the result of these errors?
Often most do not make any difference to the use and view of the site - non-fatal errors. When using mods, and even templates, you will always get errors in the error log - everyone gets these.
If it doesnt affect people I would not worry about it. The only other time to worry is if there are thousands of errors each day - then it could affect your server limits.
I have errors in the log everyday but they are ... 'normal' and expected on a modded site.
If not a file permission setting or htaccess file it is most likely caused by other mods. I think the tabs and pages ones are very old and the mods that stop spammers can result in odd effects. Im sure, if you set up a clean SMF site with only simple portal there will be no problem. After that install what is only necessary ... one by one and test.