Getting Fatalerror: Call to a member function getFrontendClass() on a non-object in /legacy/libraries/Template.php line 1513

Just before that line, can you add:

if ( ! is_object($addon))
{
    print_r($this->tag_data[$i]['class']);exit;
}

And cut and paste the output here? I suspect there is an add-on installed in the database that you may not have in your user folder.

Thanks for the heads up, it was an addon related issue.

I’ve just experienced this same error when updating an MSM site from EE5 to EE6. For me it turned out to be a missing Add-On which the Template was trying to call. I could see from the tags in the Template which Add-On was causing the problem. But I could also see the same issue highlighted in the new “Template Debug Tags” section in EE6.

Tools > Template Debug Tags
/cp/utilities/debug-tools/debug-tags

Hope this helps.