For Drupal 8.8.2, I've got an exposed filter of grouped items. Ever since the last update to 8.8.2, I've been getting an "illegal choice" error when a user navigates to the page with this filter. Once the filter is used, the error goes away. But it's back every time they reload.
It happens regardless of whether 'remember choice' is on or off.
Looking at the logs, I don't get enough information. it will just say:
Illegal choice in Type element.
Illegal choice 10,11,12,13,14 in Type element.
Even if I completely remove the choices, I get an error when nothing but -any- is an option. So it's not the specific choices that are the issue.
I turned on Devel, with kint, in hopes of finding more info and it doesn't give me any more detail.
I also tried adding the settings.local.php for verbose errors. It gave me a 502 error, so I dropped this into the settings.php, and got just the basic error.
$config['system.logging']['error_level'] = 'verbose';
Here are a couple screen shots of the filter's settings: