Error: Cannot create references to/from string offsets in Drupal\Component\Utility\NestedArray::setValue() (line 155 of /app/web/core/lib/Drupal/Component/Utility/NestedArray.php)

We had a similar problem. It has to do with Step #8. Allow multiple selections and exposed filters
We had an expose filter on a taxonomy term.

• With multiple selection unchecked the query string looks like /event?type=25 (with taxonomy term 25 selected).
• We later change the expose filter to multiple selections. Going to the view page and selecting term 25, the query string looks like this /event?type%5B25%5D=25.
• With it still selected with multiple sections and you visit /event?type=25 you get this error. So any old URL now throw an error.

So multiple section changes the query string from a value to an array. In my last comment type%5B25%5D=25 actually equals type[25]=25. If you have 2 values selected its type[25]=25&type[26]=26. So the problem is Drupal is looking for an array but finds a single value instead, and throws an error.

Just to throw some more into the mix.

I'm building a pretty standard site at the moment and have exposed a node add form to anonymous users that then throws this error when they create a node. So seemingly nothing to do with Views or a multiple filter, so could it be something else?

I tried adding the patch from https://www.drupal.org/project/drupal/issues/3023924 🐛 Cannot create references to/from string offsets Needs work , and while this did allow the submission to go through without the site-breaking error, it did not appear to allow the submitting values through, as it came back saying that all of the required fields needed to be filled out.