I found the cause of the warning:
Undefined variable $userId in /administrator/components/com_spsimpleportfolio/views/items/tmpl/default.php
The view template uses the variable $userId several times, but it was never initialized.
Adding the following line solved the issue on my installation:
$userId = (int) $user->id;
After this adjustment, the warning no longer appears for non–SuperUser backend accounts.
If possible, could you please include this initialization in a future update of the component?
It would help prevent the warning on Joomla 4/5 installations running PHP 8+.