In T335073#8869668, @Bawolff wrote:

In T335073#8869536, @Galahad wrote:

In the case of zip downloads, how would it be? I didn't download the core with composer and although I applied the patch that Taavi recommended it doesn't work. Should I include the file "composer.local.json" then?

This issue should not have applied to zip downloads. If you were using zip and got it it probably has a different cause. (Maybe from an extension?)

Observed the same here on my wiki. Specifically, I noticed it because I observed error 500 when searching due to autocomplete not being able to reach the needed resource, and confirmed by navigating to the rest.php page directly which would give a message similar to what Ethnopunk posted above. I went through the dependancy chain and eventually traced it back to psr/httpmessage version 2.0 being upgraded.